CN111344445A

CN111344445A - Precursor stabilization process

Info

Publication number: CN111344445A
Application number: CN201780096765.1A
Authority: CN
Inventors: 斯蒂芬·保罗·阿特基斯; 马克西姆·罗伯特·马格
Original assignee: Deakin University
Current assignee: Deakin University
Priority date: 2017-10-10
Filing date: 2017-10-10
Publication date: 2020-06-26
Anticipated expiration: 2037-10-10
Also published as: AU2017435381A1; JP2021500481A; US20240125009A1; JP7059496B2; CN111344445B; EP3695035A1; KR102530253B1; AU2017435381B2; CA3078394A1; KR102415617B1; KR20220093397A; CN116716684A; EP3695035A4; WO2019071286A1; US20200248339A1; US11873584B2; KR20200066646A; AU2022287549A1

Abstract

The present invention relates to an improved process for forming a stabilized precursor suitable for making carbon materials such as carbon fibers. The process can convert polyacrylonitrile containing precursors to stabilized precursors with greater efficiency. The invention also relates to a process for preparing carbon fibers using the stabilized precursor.

Description

Precursor stabilization process

Technical Field

The present invention relates to a process for forming a stabilized precursor that can be used to make carbon-based materials such as carbon fibers; and to stabilized precursors formed by the process. The invention also relates to a process for preparing carbon fibers using the stabilized precursor.

Background

Carbon fibers are fibers that primarily include carbon atoms, which are made by converting organic precursors, such as Polyacrylonitrile (PAN) precursors, to carbon.

Conventionally, carbon fibers are manufactured by subjecting a PAN precursor to a series of heat treatments which can be roughly divided into two main steps: stabilization and carbonization. The first main step, called stabilization, consists in heating the PAN precursor in air at a temperature from 200 to 300 ℃ in order to prepare a precursor capable of withstanding the subsequent carbonization step. During carbonization, the stabilized precursor undergoes chemical rearrangement, resulting in the release of non-carbon-containing atoms and the formation of highly ordered carbon-based structures. The carbonization step is typically carried out at a temperature in the range of from 400 ℃ to 1600 ℃ in a furnace containing an inert atmosphere.

The stabilization process is typically performed in a series of ovens and may take many hours to complete. Thus, precursor stabilization is costly from a time and energy perspective, thus making it an expensive part of the carbon fiber manufacturing process. Furthermore, the exothermic nature of the stabilization reaction and the combination of heat and oxygen for precursor stabilization can present a fire risk, thus creating serious safety issues.

It would be desirable to provide a process for preparing a stabilized PAN precursor that overcomes or ameliorates one or more disadvantages of conventional precursor stabilization processes. It would also be desirable to provide a process that enables carbon fibers to be manufactured in a more efficient manner.

The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

Summary of The Invention

The present invention relates to a process for preparing a stabilized precursor that can be used to make carbon materials such as carbon fibers. Advantageously, the process of the present invention may enable the rapid formation of stabilized precursor fibers useful in the manufacture of carbon fibers.

In one aspect, the present invention provides a process for preparing a stabilized precursor, the process comprising:

heating a precursor comprising a polyacrylonitrile in a substantially oxygen-free atmosphere while applying a substantially constant amount of tension to the precursor to promote cyclization of nitrile groups in the precursor, the temperature and time period at which the precursor is heated in the substantially oxygen-free atmosphere and the amount of tension applied to the precursor each being selected to form a pre-stabilized precursor having at least 10% cyclized nitrile groups as determined by fourier transform infrared (FT-IR) spectroscopy; and

the pre-stabilized precursor is exposed to an oxygen-containing atmosphere to form a stabilized precursor.

The temperature, time and tension conditions selected for the process may enable the production of a pre-stabilized precursor having at least 10% cyclized nitrile groups in a short period of time.

In particular embodiments, the temperature at which the precursor is heated in the substantially oxygen-free atmosphere and the amount of tension applied to the precursor while the precursor is heated are each selected to promote the formation of at least 10% of cyclized nitrile groups in the precursor over a time period selected from the group consisting of less than 5 minutes, less than 4 minutes, less than 3 minutes, or less than 2 minutes. Thus, the precursor need only be heated in a substantially oxygen-free atmosphere for a period of several minutes to produce a pre-stabilized precursor having at least 10% cyclized nitrile groups.

During the precursor stabilization process, the precursor is heated in a substantially oxygen-free atmosphere at a temperature sufficient to trigger the formation of at least 10% of cyclized nitrile groups in the precursor within a selected period of time.

In some embodiments, the precursor is heated in a substantially oxygen-free atmosphere at a temperature near the degradation temperature of the precursor. In a preferred embodiment, the precursor is heated in a substantially oxygen-free atmosphere at a temperature not exceeding 30 ℃ below the degradation temperature of the precursor.

In a particular embodiment, the precursor is heated in a substantially oxygen-free atmosphere at a temperature in the range of from about 250 ℃ to 400 ℃, preferably at a temperature in the range of from about 280 ℃ to 320 ℃.

The amount of tension applied to the precursor can affect the degree of cyclization of the nitrile group. The tension may be selected to enable formation of a desired amount of cyclized nitrile groups in the pre-stabilized precursor under selected parameters of temperature and time period for heating the precursor in a substantially oxygen-free atmosphere.

In one or more embodiments, the amount of tension applied to the precursor is selected to form a pre-stabilized precursor having at least 15% cyclized nitrile groups, preferably at least 20% cyclized nitrile groups as determined by fourier transform infrared (FT-IR) spectroscopy.

In a specific embodiment, the amount of tension applied to the precursor is selected to form a pre-stabilized precursor having 20% to 30% cyclized nitrile groups as determined by fourier transform infrared (FT-IR) spectroscopy.

It has been found that precursors comprising polyacrylonitrile have the potential to achieve a maximum amount of cyclisation of the nitrile groups. The pre-stabilization process parameters of temperature, time and tension may be selected to promote the maximum degree of cyclization of the nitrile group in the precursor. Alternatively, the pre-stabilization process parameters of temperature, time and tension may be selected to promote a degree of cyclization of the nitrile group in the precursor that is within an acceptable amount of the maximum amount potentially achievable.

In one embodiment, the amount of tension applied to the precursor is selected to promote a degree of cyclization of the nitrile group that is up to 80% less than the maximum amount achievable in the precursor.

In another embodiment, the amount of tension applied to the precursor is selected to promote cyclization of the nitrile group to the maximum amount achievable in the precursor. A pre-stabilized precursor having a maximum amount of cyclized nitrile groups can facilitate the formation of a stabilized precursor with improved efficiency.

In one or more embodiments, an amount of tension in a range from about 50cN to about 50,000cN is applied to the precursor when the precursor is heated in a substantially oxygen-free atmosphere.

The substantially oxygen-free atmosphere used in the precursor stabilization processes described herein may comprise a suitable gas. In one embodiment, the substantially oxygen-free atmosphere comprises nitrogen.

The pre-stabilized precursor formed according to the process of the present invention is exposed to an oxygen-containing atmosphere to form a stabilized precursor. Desirably, the stabilized precursor can be carbonized to form a carbon-based material such as carbon fiber.

The pre-stabilized precursor may only require exposure to an oxygen-containing atmosphere for a relatively short period of time to form a stabilized precursor, as compared to conventional precursor stabilization processes known in the art. In one embodiment, the pre-stabilized precursor is exposed to an oxygen-containing atmosphere for a period of time not exceeding about 30 minutes.

The pre-stabilized precursor is preferably heated while in an oxygen-containing atmosphere. Heating the pre-stabilized precursor may facilitate rapid formation of the stabilized precursor. In some particular embodiments, the pre-stabilized precursor is heated in an oxygen-containing atmosphere at a temperature in the range from about 200 ℃ to 300 ℃.

In one set of embodiments, the pre-stabilized precursor is heated in an oxygen-containing atmosphere at a temperature below the temperature used to form the pre-stabilized precursor.

Since the temperature used to form the stabilized precursor may be lower than the temperature used to form the pre-stabilized precursor, some embodiments of the precursor stabilization processes described herein may further include the step of cooling the pre-stabilized precursor prior to exposing the pre-stabilized precursor to the oxygen-containing atmosphere.

The precursor stabilization process of the present invention may enable the rapid formation of a suitably stabilized precursor.

In some embodiments, the process of the present invention may enable the formation of a stabilized precursor within a time period selected from the group consisting of no more than about 60 minutes, no more than about 45 minutes, no more than about 30 minutes, and no more than about 25 minutes.

In some embodiments, the stabilization process of the present invention can form a stabilized precursor with an average energy consumption in the range of from about 1.1kWh/kg to 2.6 kWh/kg.

One or more embodiments of the processes described herein can further include the step of determining a tension parameter of the precursor prior to forming the pre-stabilized precursor, wherein determining the tension parameter of the precursor comprises:

(a) selecting a temperature and a time period for heating the precursor in a substantially oxygen-free atmosphere;

(b) applying a series of different substantially constant amounts of tension to the precursor while heating the precursor in a substantially oxygen-free atmosphere at a selected temperature and for a selected period of time;

(c) determining the amount of cyclized nitrile groups formed in the precursor for each substantially constant amount of tension applied to the precursor by fourier transform infrared (FT-IR) spectroscopy;

(d) the tendency of the degree of cyclization of the nitrile group (% EOR) with respect to the strain was calculated,

(e) determining from the calculated trend the amount of strain in the precursor that provides at least 10% nitrile group cyclization and maximum nitrile group cyclization; and

(f) the amount of tension that causes at least 10% of the nitrile groups to cyclize is selected to pre-stabilize the precursor.

In some embodiments of the tonicity parameter determining step, the amount of tonicity that causes maximum nitrile cyclization is selected to pre-stabilize the precursor as described herein.

The above embodiments of the stabilization process may be applied to precursor fibers and incorporated into the process for making carbon fibers.

In another aspect, the present invention provides a process for preparing carbon fibers, the process comprising the steps of:

providing a stabilized precursor fiber prepared according to the process of any of the embodiments described herein; and

carbonizing the stabilized precursor fiber to form a carbon fiber.

Conventional carbonization process conditions may be employed to convert the stabilized precursor into carbon fibers. In one set of embodiments, carbonizing the stabilized precursor includes heating the stabilized precursor in an inert atmosphere at a temperature in a range from about 350 ℃ to 3000 ℃.

In one or more embodiments of the carbon fiber production process described herein, the carbon fibers are formed within a time period of no more than about 70 minutes or no more than about 45 minutes.

In some embodiments, the carbon fiber production process is continuous and comprises the steps of:

feeding a precursor comprising polyacrylonitrile to a pre-stabilization reactor comprising a substantially oxygen-free atmosphere, and heating the precursor in the substantially oxygen-free atmosphere while applying a substantially constant amount of tension to the precursor to promote cyclization of nitrile groups in the precursor, the temperature and period of time at which the precursor is heated in the substantially oxygen-free atmosphere and the amount of tension applied to the precursor each being selected to form a pre-stabilized precursor having at least 10% cyclized nitrile groups as determined by fourier transform infrared (FT-IR) spectroscopy;

feeding the pre-stabilized precursor to an oxidation reactor comprising an oxygen-containing atmosphere and exposing the pre-stabilized precursor to the oxygen-containing atmosphere to form a stabilized precursor; and

the stabilized precursor is fed into a carbonization unit, and the stabilized precursor is carbonized in the carbonization unit to form carbon fibers.

In some embodiments of the continuous carbon fiber production process described herein, there may be an additional step of cooling the pre-stabilized precursor prior to feeding the pre-stabilized precursor to the oxidation reactor.

Also provided are stabilized precursors made by the precursor stabilization process of any of the embodiments described herein.

Further, a carbon fiber produced by the carbon fiber production process of any of the embodiments described herein is also provided.

In another aspect, there is also provided a low density, stabilized precursor comprising polyacrylonitrile, having at least 60% cyclized nitrile groups and having a nitrile number of from about 1.30g/cm³To 1.33g/cm³Mass density in the range of (1).

Brief Description of Drawings

Embodiments of the present invention will now be described with reference to the following non-limiting drawings, in which:

fig. 1 shows a schematic of the pre-stabilization process of an embodiment of an aspect of the present invention performed in a furnace having one reaction chamber comprising four temperature zones operating as a pre-stabilization reactor.

Fig. 2 shows the FT-IR spectra of an untreated PAN precursor and a pre-stabilized PAN precursor fiber (PSN-1) according to an embodiment of the process of an aspect of the invention, which pre-stabilized PAN precursor fiber (PSN-1) has been heated under an applied tension of 3,000cN in a nitrogen atmosphere.

Fig. 3 shows the FT-IR spectrum of a pre-stabilized PAN precursor fiber (PSN-2) of an embodiment of the process according to an aspect of the invention, which pre-stabilized PAN precursor fiber (PSN-2) has been heated under an applied tension of 2300cN in a nitrogen atmosphere.

Fig. 4 shows DSC curves illustrating the heat flow of an untreated PAN precursor and a pre-stabilized PAN precursor fiber heated under applied tension of 2,300cN, 2,500cN, and 3,000cN in a nitrogen atmosphere (labeled as PSN-3, PSN-4, and PSN-5, respectively) according to an embodiment of the process of an aspect of the invention.

Fig. 5 shows a schematic of an oxidation process of an embodiment of one aspect of the present invention conducted in a reactor having four oxidation chambers providing four temperature zones, with multiple passes of the pre-stabilized precursor through each temperature zone.

Fig. 6 shows FT-IR spectra of a stabilized PAN precursor (PSN OPF) and a comparative stabilized PAN precursor (baseline OPF) produced by oxidizing a pre-stabilized precursor according to the process illustrated in fig. 5.

Fig. 7 shows a schematic of an oxidation process of an embodiment of one aspect of the present invention conducted in a reactor having two oxidation chambers providing two temperature zones, wherein the pre-stabilized precursor passes through each temperature zone in multiple passes.

Fig. 8 shows the FT-IR spectrum of a stabilized PAN precursor fiber produced by oxidizing a pre-stabilized precursor according to the process illustrated in fig. 7.

Fig. 9 shows a schematic of an oxidation process of an embodiment of one aspect of the present invention conducted in a reactor having a single oxidation chamber providing a single temperature zone through which a pre-stabilized precursor is passed in multiple passes.

Fig. 10 shows the FT-IR spectrum of a stabilized PAN precursor fiber produced by oxidizing a pre-stabilized precursor according to the process illustrated in fig. 9.

Fig. 11 shows a schematic of the oxidation process of an embodiment of an aspect of the present invention conducted in a reactor having four oxidation chambers providing four temperature zones, with a single pass of the pre-stabilized precursor through each temperature zone.

Fig. 12 shows FT-IR spectra of pre-stabilized PAN precursor fibers after oxidation in each of the temperature zones shown in fig. 11.

Fig. 13 shows a graph illustrating the change in% EOR with applied tension for a stabilized commercial PAN precursor (precursor a) having an elliptical cross-sectional shape in the form of a 50K tow coated with sizing according to an embodiment of an aspect of the invention.

Fig. 14 shows a graph illustrating the change in mass density of an embodiment stabilized commercial PAN precursor (precursor a) with applied tension according to an aspect of the invention.

Fig. 15 shows a graph illustrating the change in tensile modulus and ultimate tensile strength of an embodiment stabilized commercial PAN precursor (precursor a) with applied tension according to an aspect of the invention.

Fig. 16 shows a graph illustrating the change in% EOR with applied tension of an embodiment stabilized commercial PAN precursor (precursor B) having a circular cross-sectional shape in the form of a tow comprising 24,000 filaments (1.6 dtex) coated with a silicon-based sizing agent, according to an aspect of the invention.

Fig. 17 shows a graph illustrating the change in mass density of an embodiment stabilized commercial PAN precursor (precursor B) with applied tension according to an aspect of the invention.

Fig. 18 shows a graph illustrating the change in tensile modulus and ultimate tensile strength of an embodiment stabilized commercial PAN precursor (precursor B) with applied tension according to an aspect of the invention.

FIG. 19 shows a graphical representation of dehydrogenation index (CH/CH)₂Ratio) as a function of the heating dwell time (dwell in heat) of the stabilized precursor fibers of embodiments of the invention formed after oxidative stabilization of pre-stabilized precursor fibers having different nitrile group cyclization contents (% EOR) of 17%, 24%, and 28%.

Fig. 20 shows a graph illustrating the degree of nitrile group cyclization as a function of heating residence time for a stabilized precursor fiber of an embodiment of the invention formed after oxidative stabilization of pre-stabilized precursor fibers having different nitrile group cyclization contents (% EOR) of 17%, 24%, and 28%.

Figure 21 shows DSC traces of different PAN precursors under nitrogen atmosphere to illustrate the degradation temperatures of the precursors.

Fig. 22 shows a block diagram of a carbon fiber production system for performing the continuous carbon fiber production process of an embodiment of the present invention.

Detailed Description

As used herein, the singular forms "a", "an" and "the" refer to both the singular and the plural, unless expressly stated otherwise.

The use of the terms "about" and the general ranges, whether or not defined by the terms about, means that the numbers understood are not limited to the exact numbers set forth herein, and are intended to refer to ranges substantially within the recited ranges without departing from the scope of the invention. As used herein, "about" will be understood by one of ordinary skill in the art and will vary to some extent in the context in which the term is used. If the use of a term is not clear to one of ordinary skill in the art in view of the context in which the term is used, "about" will mean up to plus or minus 10% of the particular term.

In general terms, the present invention provides a process for preparing a stabilized precursor that can be used to make carbon-based materials, particularly carbon fibers. The stabilization process described herein includes a pre-stabilization step that forms a pre-stabilized precursor. It has been found that including a pre-stabilization step can help improve the efficiency of the process used to form the stabilized precursor.

In particular, it has been found that a stabilization process comprising a pre-stabilization step as described herein enables the formation of stabilized precursors suitable for the manufacture of carbon fibers in a fast manner.

In one aspect, the present invention provides a process for preparing a pre-stabilized precursor, the process comprising: heating a precursor comprising a polyacrylonitrile in a substantially oxygen-free atmosphere while applying a substantially constant amount of tension to the precursor to promote cyclization of nitrile groups in the precursor, the temperature and time period at which the precursor is heated in the atmosphere and the amount of tension applied to the precursor each being selected to form a pre-stabilized precursor having at least 10% cyclized nitrile groups as determined by fourier transform infrared (FT-IR) spectroscopy.

A pre-stabilized precursor having at least 10% cyclized nitrile groups as described herein is intended to mean a partially stabilized precursor that can be further processed in an oxygen-containing atmosphere to form a stabilized precursor. The stabilized precursor so formed may be suitably carbonized to form a carbon-based material.

It has been found that by initiating a stabilization reaction in a substantially oxygen-free atmosphere by heating the precursor in a substantially oxygen-free atmosphere at a selected temperature for a selected period of time and while a selected substantially constant amount of tension is applied to the precursor, a pre-stabilized precursor having at least 10% cyclized nitrile groups can be formed that is activated for subsequent reaction in an oxygen-containing atmosphere. After exposing the pre-stabilized precursor to an oxygen-containing atmosphere, the stabilized precursor can then be formed easily and quickly.

An important part of the present invention is that a pre-stabilized precursor having at least 10% of cyclized nitrile groups is formed by heating a precursor comprising a polyacrylonitrile in a substantially oxygen-free atmosphere. Without wishing to be bound by theory, it is believed that by forming at least 10% of the cyclized nitrile groups in the pre-stabilized precursor, downstream advantages can be imparted to the oxidative precursor stabilization and carbonization of the oxidatively stabilized precursor to form carbon-based materials of acceptable quality (including high performance quality), such as carbon fibers. In particular, it is believed that a pre-stabilized precursor having at least 10% cyclized nitrile groups can facilitate faster, safer, and lower cost stabilization of the precursor and formation of carbon fibers. It is also believed that the benefits provided by the process of the present invention, such as high speed formation of a suitably stabilized precursor that can be converted into a carbon-based material such as carbon fiber, improved safety of precursor stabilization, and reduction in energy consumption, are not realized when less than 10% cyclization of nitrile groups is obtained in the pre-stabilized precursor.

The stabilized precursors described herein formed according to the stabilization process of the present invention are thermally stable. By "thermally stable" is meant that the stabilized precursor is resistant to combustion or degradation when exposed to an open flame, and may be suitably carbonized to form a carbon-based material such as carbon fiber.

The stabilized precursor formed by the stabilization process of the present invention may also be referred to herein as a "fully stabilized precursor". This is in contrast to the pre-stabilized precursors described herein, which are partially stabilized precursors.

By subjecting the precursor to an initial pre-stabilization and forming a pre-stabilized precursor having at least 10% cyclized nitrile groups as described herein, a stabilized precursor suitable for the manufacture of carbon-based materials, such as carbon fibers, can be produced with improved efficiency.

The processes described herein can facilitate rapid formation of stabilized precursors and help accelerate the precursor stabilization step used in carbon fiber manufacture. Furthermore, the processes described herein can help reduce the costs associated with the precursor stabilization step, as well as help improve the safety of precursor stabilization.

Precursor body

The process of the present invention can be used for the stabilization of precursors comprising Polyacrylonitrile (PAN). Precursors comprising PAN are also referred to herein as "polyacrylonitrile precursors" or "PAN precursors".

Reference herein to PAN precursors includes precursors comprising homopolymers of acrylonitrile as well as copolymers and terpolymers of acrylonitrile with one or more comonomers.

Thus, the term "polyacrylonitrile" as used herein includes homopolymers, copolymers and terpolymers formed at least by polymerization of acrylonitrile. Such polymers are generally linear and will have nitrile groups pendant from the carbon-based polymer backbone.

As will be discussed further below, cyclisation of the pendant nitrile group will play an important part of the invention.

The precursor for use in the present invention may comprise polyacrylonitrile having at least about 85% acrylonitrile units by weight. In some embodiments, the precursor used herein may comprise polyacrylonitrile having less than 85% acrylonitrile units by weight. Such polymers may include modacrylic polymers, which are generally defined as polymers containing from 35% to 85% by weight acrylonitrile units and which are typically copolymerized with vinyl chloride or vinylidene chloride.

Polyacrylonitrile (PAN) is a suitable polymer for inclusion in precursors for the production of carbon-based materials, such as carbon fibers, due to its physical and molecular properties and its ability to provide high carbon yields.

In one set of embodiments, the precursors employed in the process of the present invention may comprise polyacrylonitrile homopolymers, polyacrylonitrile copolymers, or mixtures thereof.

One skilled in the relevant art will appreciate that polyacrylonitrile homopolymers are polymers that include polymerized units derived solely from acrylonitrile.

Polyacrylonitrile copolymers are copolymers of acrylonitrile with at least one comonomer. Examples of comonomers include acids such as itaconic acid and acrylic acid, ethylenically unsaturated esters such as vinyl acetate, methyl acrylate and methyl methacrylate, ethylenically unsaturated amides such as acrylamide and methacrylamide, ethylenically unsaturated halides such as vinyl chloride, and sulfonic acids such as vinyl sulfonate and p-styrene sulfonate. The polyacrylonitrile copolymer may comprise from 1 to 15% by weight or from 1 to 10% by weight of one or more comonomers. The precursor may comprise two or more different types of PAN copolymers.

The polyacrylonitrile in the precursor may have a molecular weight of at least 200 kDa.

The chemical mechanism involved in the stabilization of polyacrylonitrile precursors in preparation for carbonization is not well understood. However, it is believed that cyclization of the pendant nitrile groups on the acrylonitrile units in the polyacrylonitrile polymer can play an important role in forming sufficiently stabilized precursors that can withstand the high temperature conditions used for carbonization.

Cyclization of the pendant nitrile groups in the polyacrylonitrile polymer produces a hexagonal carbon-nitrogen ring as shown below:

heat and gases (such as HCN gas) are typically generated as a result of the cyclization of nitrile groups.

In one set of embodiments, the precursor may be a polyacrylonitrile copolymer of acrylonitrile and at least one acidic comonomer. Examples of acidic comonomers include acids such as itaconic acid and acrylic acid. The polyacrylonitrile copolymer may comprise from 1 to 15% by weight or from 1 to 10% by weight of polymerized units derived from at least one acidic comonomer.

In some embodiments, it is preferred to utilize a precursor of a polyacrylonitrile copolymer comprising acrylonitrile and at least one acidic comonomer as a feedstock for the stabilization process of the present invention. It is believed that the polymerized units derived from the acidic comonomer may become deprotonated, thereby catalyzing the cyclization of the nitrile group in the precursor. Thus, initiation of the cyclization of the nitrile group can occur at lower temperatures. The inclusion of polymerized units derived from acidic comonomers in polyacrylonitrile can also help control the exotherm resulting from the cyclization of the nitrile group.

In the precursor of the polyacrylonitrile copolymer comprising acrylonitrile and at least one acidic comonomer, the cyclic group formed during the stabilization of the precursor may have the structure as shown below:

in one set of embodiments, the precursor employed in the process of the present invention may comprise polyacrylonitrile mixed or blended with another substance.

In some embodiments, the additional substance may be an additional polymer. In such embodiments, the blend or mixture preferably comprises at least 50% by weight of Polyacrylonitrile (PAN). The PAN is mixed with at least one additional polymer.

In embodiments where the precursor comprises polyacrylonitrile blended or mixed with at least one additional polymer, the weight ratio of PAN to additional polymer in the precursor may be selected from 55:45, 60:40, 70:30, 80:20, 85:15, 90:10, and 95: 5.

The polyacrylonitrile in the blend or mixture may be a polyacrylonitrile homopolymer or a polyacrylonitrile copolymer, as described herein.

The polyacrylonitrile copolymer may comprise at least 85% by weight or at least 90% by weight of polymerized units derived from acrylonitrile. The remainder of the polymerized units in the polyacrylonitrile copolymer are derived from one or more comonomers, such as acidic comonomers.

In some embodiments of the mixtures and blends mentioned herein, the additional polymer may be selected from polymers known for use in the manufacture of carbon fibers. In some embodiments, the additional polymer may be selected from the group consisting of: petroleum pitch, thermoplastic polymers, cellulose, rayon, lignin, and mixtures thereof. Thermoplastic polymers may include, but are not limited to, Polyethylene (PE), poly (ethylene terephthalate) (PET), poly (butylene terephthalate) (PBT), polypropylene (PP), poly (vinyl chloride) (PVC), poly (vinylidene fluoride) (PVDF), Polycarbonate (PC), polyphenylene oxide (PPO), and poly (styrene) (PS).

In some embodiments, the precursor may comprise polyacrylonitrile mixed or blended with fillers, such as nanofillers. Exemplary nanofillers may be carbon nanoparticles, such as carbon nanotubes or graphene nanoparticles.

In some embodiments, the precursor may be surface treated. For example, the precursor may comprise an optional surface coating (i.e., a sizing or spin finish). The presence of the surface treatment does not detract from the benefits of the invention.

The precursors employed in the process of the present invention may be in a range of forms including, but not limited to, fiber forms, yarn forms, web forms, film forms, fabric forms, woven forms, felt forms, and mat forms. The pad may be a woven or non-woven pad.

The precursor is preferably in the form of a continuous length of material, such as a continuous length of fibre. The precursor fiber may comprise a bundle of filaments.

The precursor may also have a different cross-sectional configuration including, for example, a circular, oval, bean, dog bone, petal or other shaped cross-section. The precursor may be hollow and have one or more internal voids. The internal voids may be continuous or discontinuous.

In one set of embodiments, the precursor is in the form of a fiber, preferably a continuous fiber. Many PAN precursor fibers are known and commercially available. The process of the present invention can be used to stabilize a variety of PAN precursors from both commercial and non-commercial sources.

The PAN precursor fibers may be provided in one or more tows, each tow having fibers comprising a plurality of continuous filaments. The tow containing the PAN precursor may be in a variety of sizes, with the size depending on the number of filaments per tow. For example, a tow may contain between 100 and 1,000,000 filaments per tow. This corresponds to a tow size of from about 0.1K to about 1,000K. In some embodiments, the tow may comprise from 100 to 320,000 filaments per tow, which corresponds to a tow size of from about 0.1K to about 320K.

The filaments forming the PAN precursor fiber may have a range of diameters. For example, the diameter may range from about 1 micron to 100 microns, about 1 micron to 30 microns, or 1 micron to 20 microns. However, the magnitude of such diameter is not critical to the process of the present invention.

Precursor stabilization

The stabilization process of the present invention involves two precursor treatment stages, namely pre-stabilization and oxidation, to form a stabilized precursor. These two phases are discussed further below.

In one aspect, the present invention provides a process for preparing a stabilized precursor, the process comprising the steps of:

a pre-stabilization stage comprising heating a precursor comprising a polyacrylonitrile in a substantially oxygen-free atmosphere while applying a substantially constant amount of tension to the precursor to promote cyclization of nitrile groups in the precursor, the temperature and time period at which the precursor is heated in the substantially oxygen-free atmosphere and the amount of tension applied to the precursor each being selected to form a pre-stabilized precursor having at least 10% cyclized nitrile groups as determined by fourier transform infrared (FT-IR) spectroscopy; and

an oxidation stage comprising exposing the pre-stabilized precursor to an oxygen-containing atmosphere.

For convenience, in the process described below, reference to a precursor means a precursor in the form of a fiber. However, it will be understood that the process may be applied to other forms of precursors, such as the yarn forms, web forms and mat forms described above, and is not limited to fiber forms.

Pre-stabilisation

To form a stabilized precursor, the process described herein includes the step of heating a precursor fiber in a substantially oxygen-free atmosphere while applying a substantially constant amount of tension to the precursor. As a result of this step, a pre-stabilized precursor fiber is thus produced. This step of the precursor stabilization process described herein may also be referred to as a "pre-stabilization" or "pre-stabilization" step. Thus, the pre-stabilization step converts the PAN precursor into a pre-stabilized precursor.

The terms "pre-stabilization" and "pre-stabilization" as used herein with respect to a step of the stabilization process described herein indicate that this step is a preparative step that occurs prior to complete stabilization of the precursor in the oxidation step described below. Thus, the pre-stabilization step may be considered as a pre-treatment step or pre-oxidation step, which subjects the precursor to a preliminary treatment prior to complete stabilization in the oxidation step. Thus, the process of the present invention includes the step of pre-treating the precursor to aid in the preparation of the precursor for oxidative stabilization in an oxygen-containing atmosphere as discussed below. Thus, the term "pre-stabilized precursor" indicates a precursor that has undergone the "pre-stabilization" treatment described herein.

The pre-stabilization step described herein may advantageously facilitate rapid and efficient conversion of the precursor to a stabilized precursor by enabling the initial formation of a partially stabilized precursor that is activated for oxidative stabilization. When the stabilized precursor is carbonized to form a carbon-based material, such as carbon fibers, rapid formation of the stabilized precursor can impart downstream advantages, as discussed below. Downstream benefits may be particularly advantageous in a continuous process for manufacturing materials such as carbon fibers.

A substantially oxygen-free atmosphere is used for the pre-stabilization step. The term "substantially oxygen-free atmosphere" means an atmosphere substantially free of oxygen atoms. The oxygen atom may be an oxygen-containing molecule such as molecular oxygen (i.e., O) in the atmosphere₂) Or water (i.e. H)₂O) is used. However, the term "substantially oxygen-free atmosphere" will allow the presence of oxygen atoms that form part of the molecular structure of the polymer in the precursor.

It is preferred to limit the amount of oxygen atoms in the substantially oxygen-free atmosphere, as it is believed that oxygen atoms can adversely affect the rate of cyclization of nitrile groups, and thus the ability to achieve the necessary amount of cyclized nitrile groups in the pre-stabilized precursor over a selected period of time.

Thus, an important part of the process is that the pre-stabilization and the formation of a pre-stabilized precursor comprising at least 10% of cyclized nitrile groups is carried out in a substantially oxygen-free atmosphere.

It is desirable that water (e.g., in the form of steam or water vapor) is not present in the substantially oxygen-free atmosphere, as water can cause cooling of the atmosphere. Therefore, more energy will need to be consumed in order to maintain the substantially oxygen-free atmosphere at the desired temperature. Thus, it is preferred that the substantially oxygen-free atmosphere used in the pre-stabilization step be at least substantially free of water, and in one preferred embodiment, free of water.

As discussed above, the term "substantially oxygen-free atmosphere" is also used to indicate that the atmosphere is substantially free of molecular oxygen (i.e., O)₂) This molecular oxygen is commonly referred to as "oxygen". Small amount of oxygen (i.e. O)₂) May be present in the atmosphere to which the precursor fiber is exposed. The substantially oxygen-free atmosphere can comprise no more than 1%, no more than 0.5%, no more than 0.1%, no more than 0.05%, no more than 0.01%, or no more than 0.005% oxygen (O) by volume₂). In some embodiments, it is preferred that no oxygen is present, such that the atmosphere used during pre-stabilization is oxygen-free.

It may be desirable to limit the amount of oxygen in the substantially oxygen-free atmosphere, as the presence of oxygen may pose a fire risk at some operating temperatures for forming the pre-stabilized precursor.

In one set of embodiments, the substantially oxygen-free atmosphere comprises an inert gas. Suitable inert gases may be noble gases such as argon, helium, neon, krypton, xenon, and radium. A suitable inert gas may be nitrogen. The substantially oxygen-free atmosphere may comprise a mixture of inert gases, such as a mixture of nitrogen and argon.

In one embodiment, the substantially oxygen-free atmosphere is provided by a substantially oxygen-free gas. The substantially oxygen-free gas is preferably an inert gas as described herein. In one embodiment, the substantially oxygen-free gas is nitrogen. The nitrogen gas may have a purity of 99.995% and a dew point below-30 ℃.

In some embodiments, the substantially oxygen-free gas may be medical grade nitrogen at a purity of at least 99.995%. Medical grade nitrogen is available from many commercial suppliers.

In one embodiment, the precursor is heated in a nitrogen atmosphere.

Heating of the precursor in a substantially oxygen-free atmosphere is carried out for a desired period of time and at a desired temperature. Further, a substantially constant amount of tension is applied to the precursor fibers as the precursor is heated in a substantially oxygen-free atmosphere for a desired period of time.

The temperature and time at which the precursor is heated in a substantially oxygen-free atmosphere and the amount of tension applied to the precursor during the heat treatment are each selected to promote cyclization of the nitrile groups in the precursor. Thus, the respective temperature, time and tension process conditions for the pre-stabilization step are each set to promote the formation of the desired amount of cyclized nitrile groups in the pre-stabilized precursor.

In the pre-stabilization step described herein, the temperature and time at which the precursor is heated in a substantially oxygen-free atmosphere and the tension applied to the precursor are each selected to promote and control the cyclization of the nitrile groups such that a pre-stabilized precursor is formed that comprises a desired percentage of cyclized nitrile groups. In particular, the temperature and time at which the precursor is heated in a substantially oxygen-free atmosphere and the amount of tension applied to the precursor are each selected to control the cyclization of the nitrile groups such that a pre-stabilized precursor is formed having at least 10% cyclized nitrile groups as determined by fourier transform infrared (FT-IR) spectroscopy.

In some embodiments, the temperature and time at which the precursor is heated in a substantially oxygen-free atmosphere and the amount of tension applied to the precursor are each selected to control cyclization of the nitrile groups such that a pre-stabilized precursor is formed having at least 15% or at least 20% cyclized nitrile groups as determined by fourier transform infrared (FT-IR) spectroscopy.

In other embodiments, the temperature and time at which the precursor is heated in the substantially oxygen-free atmosphere and the amount of tension applied to the precursor are each selected to control the cyclization of the nitrile groups such that a pre-stabilized precursor is formed having 10% to 50%, 15% to 45%, or 20% to 30% cyclized nitrile groups as determined by fourier transform infrared (FT-IR) spectroscopy.

The process conditions selected for the pre-stabilization step may facilitate the formation of a pre-stabilized precursor suitable for high-speed conversion to carbon fibers. That is, the temperature and time period for heating the precursor in a substantially oxygen-free atmosphere and the tension applied to the precursor are suitably selected to enable the formation of a pre-stabilized precursor having the desired properties, which can subsequently be rapidly converted into carbon fibers.

It will be appreciated that if a lower temperature or a higher temperature is desired for heating the precursor during the pre-stabilization step, appropriate adjustments may be made to the time period for heating the precursor and/or the tension applied to the precursor in view of the selected temperature. For example, if the temperature at which the precursor is heated in the substantially oxygen-free atmosphere is increased, the time period for heating the precursor may be decreased to compensate for the increased temperature, and vice versa.

A number of indicators may be used to guide the selection of process conditions (i.e., heating temperature, time period, and tension) for converting the precursor to the pre-stabilized precursor. One skilled in the art will appreciate that different PAN precursor feedstocks may have different properties. Thus, for a given precursor feedstock, the index may facilitate the selection of appropriate time, temperature and tension conditions to be used in the pre-stabilization step, such that a pre-stabilized precursor having the desired properties is formed at the end of the pre-stabilization step. These indices may be considered individually or in combination.

An indicator used to guide the selection of pre-stabilization process conditions is the degree of cyclization of the nitrile group (expressed as the degree of reaction (% EOR)). The degree of reaction (% EOR) corresponds to the percentage of cyclized nitrile groups in the pre-stabilized precursor. The skilled person will understand that the nitrile group cyclises to produce a conjugated C-N double bond structure in the PAN precursor from a C-N triple bond.

According to the method developed by Collins et al, Carbon,26(1988)671-679,% EOR can be determined using Fourier transform infrared (FT-IR) spectroscopy. Under this approach, the following formula may be used:

wherein Abs (1590) and Abs (2242) are at 1590cm^-1And 2242cm^-1The absorbance of the peaks reported here, which correspond to the C ═ N group and the nitrile (-CN) group, respectively. Nitrile group (2242 cm)^-1) Is converted to a C ═ N group by cyclization. Therefore, at 1590cm^-1And 2242cm^-1The ratio of absorbance between the peaks at (a) can provide an indication of the proportion of nitrile groups that have undergone cyclization.

The cyclization of nitrile groups as described herein is most suitably determined by fourier transform infrared (FT-IR) spectroscopy.

Thus,% EOR and% of nitrile groups cyclized (%) represent the proportion of available nitrile (-CN) groups in the polyacrylonitrile in the precursor that have been converted to C ═ N groups by cyclization.

The process conditions selected for the pre-stabilization step are sufficient to form a pre-stabilized precursor having a predetermined% EOR, in particular a% EOR of at least 10%. In some embodiments, the process conditions selected for the pre-stabilization step described herein are sufficient to form a pre-stabilized precursor having at least 15% or at least 20% cyclized nitrile groups.

It has been found that the amount of cyclized nitrile group (% EOR) in the pre-stabilized precursor can be varied by selecting specific process parameters for the pre-stabilization step. For example, in some embodiments, it has been found that the degree of cyclization of nitrile groups in the precursor can be varied by applying varying amounts of tension to the precursor fiber when the precursor is heated in a substantially oxygen-free atmosphere at fixed conditions of temperature and time.

The temperature and time period at which the precursor is heated in a substantially oxygen-free atmosphere may also affect the cyclization of the nitrile group. However, without wishing to be bound by theory, it is believed that the amount of tension applied to the precursor may exert a greater influence on the formation of the ring structure.

In particular, it has been found that the tension applied to the precursor can control the degree of cyclization of the nitrile group in the precursor. This may occur because the tension applied to the precursor may affect the molecular arrangement of the polyacrylonitrile in the precursor.

As an example, pre-stabilization of the PAN precursor may include heating the precursor comprising polyacrylonitrile at a predetermined temperature for a predetermined period of time in a substantially oxygen-free atmosphere while applying a substantially constant amount of tension to the precursor. In such embodiments involving predetermined heating temperatures and times, the amount of applied tension can affect the degree of cyclization of the nitrile group in the precursor. Thus, when the time and temperature conditions for the pre-stabilization step are fixed, applying different substantially constant amounts of tension to the precursor under these fixed conditions may result in different amounts of cyclized nitrile groups in the precursor. Thus, the applied tension may control the degree of cyclization of the nitrile groups, allowing the formation of a pre-stabilized precursor comprising a predetermined percentage of cyclized nitrile groups.

In particular embodiments, the% EOR may be adjusted by varying the amount of tension applied to the precursor during pre-stabilization. Thus, the amount of tension applied to the precursor in the pre-stabilization step can be controlled to ensure the formation of the desired amount of cyclized nitrile groups. In turn, this may facilitate the evolution of specific chemical and structural properties in the pre-stabilized fiber.

In one set of embodiments, the amount of tension applied to the PAN precursor during pre-stabilization is selected to form a pre-stabilized precursor having at least 10%, at least 15%, or at least 20% cyclized nitrile groups as determined by FT-IR spectroscopy.

In a preferred embodiment, the amount of tension applied to the precursor promotes the formation of a high content of cyclized nitrile structures in the pre-stabilized precursor.

A high content of cyclized nitrile groups can facilitate efficient processing of the precursor for forming a stabilized precursor.

In addition, the large number of cyclized nitrile groups can facilitate the rapid formation of thermally stable, partially stabilized precursors.

Theoretically, there is no upper limit to the amount of cyclized nitrile groups that may be present in the pre-stabilized precursor. However, in practice, it may be desirable for the pre-stabilized precursor to have no more than about 50%, no more than about 45%, or no more than about 35% cyclized nitrile groups.

In some embodiments, the pre-stabilized precursor may have from about 10% to about 50%, from between about 15% to about 45% cyclized nitrile groups, or from about 20% to about 30% cyclized nitrile groups, as determined by FT-IR spectroscopy.

Without wishing to be bound by theory, it is believed that the cyclization of a portion of the nitrile group present in the precursor may facilitate the preparation of the precursor for subsequent stabilization reactions in an oxygen-containing environment. Thus, a benefit provided by the pre-stabilization step is the ability to form a precursor with the desired amount of cyclized nitrile groups, which can readily undergo further reaction to form a stabilized precursor. Thus, the pre-stabilization step may allow for the formation of a stabilized precursor in less time and with less energy.

The cyclisation of the nitrile group in the precursor can be promoted by thermal initiation and thereafter by an increase in the molecular arrangement of the polyacrylonitrile in the precursor due to the applied tension. The cyclized nitrile group can form a fused hexagonal carbon-nitrogen ring in the precursor. The result is an at least partially stabilized precursor fiber and wherein at least a portion of the PAN has been transformed into a ladder structure due to the cyclized nitrile group.

The cyclization of the nitrile group in the PAN precursor is exothermic and exothermic energy is released as the nitrile group undergoes cyclization. The exothermic behavior may vary between different precursors. Thus, the heating temperature and time period selected for heating the precursor, as well as the applied tension for pre-stabilizing the precursor in a substantially oxygen-free atmosphere, may be adapted to a given precursor in order to suitably pre-stabilize the precursor and manage its exothermic behavior.

Other indicators that may also aid in the selection of appropriate process conditions for the pre-stabilization step, in addition to% EOR, include color, mechanical properties (including tensile properties such as tensile strength, tensile modulus, and elongation), mass density, and appearance of the precursor. Each of these other indicators is discussed further below.

The color of the original (untreated) PAN precursor is usually white. The PAN precursor undergoes a color change during the pre-stabilization step, which can be visually observed. It is believed that the color evolution that occurs is chemically induced due to the formation of cyclized nitrile groups in the PAN precursor. The pre-stabilized precursor having at least 10% cyclized nitrile groups, e.g., having about 20% cyclized nitrile groups, can have a color ranging from dark yellow or orange to a copper color. Thus, the change in color of the PAN precursor may assist the person skilled in the art in selecting an appropriate temperature and time period for heating the precursor. However, for production quality control purposes, while color variation can be observed, the value of% EOR must be measured to ensure that the process is within tolerance. The temperature and time period at which the precursor is heated in a substantially oxygen-free atmosphere and the tension applied to the precursor may be selected to ensure that the desired colour of the precursor is obtained at the end of the pre-stabilisation step. Preferably, the temperature and period of time at which the precursor is heated in a substantially oxygen-free atmosphere is not so high or so long that the color of the precursor becomes dark brown or black.

Another useful indicator that may help guide the selection of process conditions for the pre-stabilization step is the mechanical properties of the pre-stabilized precursor, in particular its tensile properties.

It has been found that the tensile properties of ultimate tensile strength and tensile modulus in the PAN precursor can be reduced after the pre-stabilization step. Furthermore, it has been found that the elongation of the precursor can be increased after the pre-stabilization step.

In one form of the pre-stabilization step, the temperature and period of time at which the precursor is heated in a substantially oxygen-free atmosphere and the amount of tension applied to the precursor while the precursor is heated in that atmosphere are selected so as to form a pre-stabilized precursor having an ultimate tensile strength that is lower than the ultimate tensile strength of the original PAN precursor. In one set of embodiments, the pre-stabilized precursor produced by the pre-stabilization step has an ultimate tensile strength that is up to 60% lower than the ultimate tensile strength of the original, virgin PAN precursor, e.g., from about 15% to about 60% lower.

In one form of the pre-stabilization step, the temperature and period of time at which the precursor is heated in a substantially oxygen-free atmosphere and the amount of tension applied to the precursor while it is heated in that atmosphere are selected so as to form a pre-stabilized precursor having a tensile modulus lower than the tensile modulus of the original PAN precursor. In one set of embodiments, the pre-stabilized precursor produced by the pre-stabilization step has a tensile modulus that is up to 40% lower than the tensile modulus of the original, virgin PAN precursor, e.g., from about 15% to about 40% lower.

In one form of the pre-stabilization step, the temperature and period of time at which the precursor is heated in a substantially oxygen-free atmosphere and the amount of tension applied to the precursor while it is heated in that atmosphere are selected so as to form a pre-stabilized precursor having an elongation-to-break that is higher than the elongation-to-break of the original PAN precursor. In one set of embodiments, the pre-stabilized precursor produced by the pre-stabilization step has an elongation at break that is up to 45% higher than the elongation at break of the original PAN precursor, e.g., from about 15% to about 45% higher.

Another indicator that guides the selection of pre-stabilization process conditions is the mass density of the PAN precursor. The precursor mass density may be increased after processing the precursor in a pre-stabilization step, as described herein. In one form of the pre-stabilization step, the temperature and period of time at which the precursor is heated in a substantially oxygen-free atmosphere and the amount of tension applied to the precursor while it is heated in that atmosphere are selected so as to form a precursor having a surface tension in the range of from about 1.19g/cm³To 1.25g/cm³E.g. from 1.21g/cm³To 1.24g/cm³A mass density in the range of (a).

As yet another indicator, the appearance of PAN precursors may also help guide the selection of pre-stabilization process conditions. PAN precursors that have been pre-stabilized are preferably substantially defect-free and have an acceptable appearance. It is believed that defects including melting of the precursor or partial strand breakage may result in low mechanical properties (e.g., tensile properties) or even failure of the carbon material prepared with the precursor.

The process conditions for the pre-stabilization step may be selected to ensure that the resulting pre-stabilized precursor has, in addition to the requisite% EOR, one or more properties within the parameters described above, selected from color, mechanical properties (including tensile properties selected from ultimate tensile strength, tensile modulus, and elongation at break), mass density, and appearance.

In one form of the pre-stabilization step, the temperature and period of time at which the precursor is heated in a substantially oxygen-free atmosphere and the amount of tension applied to the precursor while the precursor is heated in that atmosphere are each selected so as to form a substantially defect-free pre-stabilized PAN precursor.

The precursor fiber is heated in a substantially oxygen-free atmosphere at a temperature and for a time period sufficient to at least initiate and promote cyclization of at least 10% of the available nitrile groups in the precursor, and optionally also promote evolution of one or more of the other indicators described above.

In one set of embodiments, heating the precursor fiber in a substantially oxygen-free atmosphere is carried out for a relatively short period of time, more preferably within minutes. This may allow for the rapid formation of a pre-stabilized precursor.

Heating the precursor in a substantially oxygen-free atmosphere for a short period of time may be desirable as this may help impart downstream advantages that help improve the efficiency of precursor stabilization and subsequently also improve carbon fiber manufacture, particularly in terms of processing time. In particular, it has been found that the pre-stabilization step described herein can facilitate high speed conversion of PAN precursor fibers to carbon fibers, as it facilitates rapid formation of stabilized precursor fibers.

To enable the PAN precursor to be treated for a short period of time, parameters such as the temperature at which the precursor is heated and the amount of tension applied to the PAN precursor during the thermal treatment may be selected to ensure that the desired period of time for pre-stabilization may be met.

In one set of embodiments, the precursor is heated in a substantially oxygen-free atmosphere at a temperature sufficient to initiate at least cyclization of a portion of the nitrile groups present in the precursor. In some embodiments, heating the precursor is performed at a selected temperature for a selected period of time.

Visually, the nitrile group cyclization can be indicated by a change in color of the precursor from white to a color ranging from dark yellow to copper. It has been observed that a color change occurs even after heating the precursor in a substantially oxygen-free atmosphere for a short period of time.

It may be advantageous to subject the PAN precursor to elevated temperatures for a brief period of time while in a substantially oxygen-free atmosphere in order to trigger the cyclization of nitrile groups in the precursor during the pre-stabilization step.

In some embodiments, the temperature selected for heating the precursor in a substantially oxygen-free atmosphere is sufficiently high to trigger or initiate cyclization of the nitrile groups in the PAN precursor, but not so high that the physical integrity of the precursor is compromised (e.g., the precursor fiber melts, breaks, or degrades). For example, it is desirable that the PAN precursor is heated at a temperature not higher than the degradation temperature of the precursor. At the same time, as a minimum, the PAN precursor should be heated at a temperature sufficient to induce cyclization of nitrile groups in the precursor during the desired processing time period when in a substantially oxygen-free atmosphere.

In some embodiments, during the pre-stabilization step, the PAN precursor is heated in a substantially oxygen-free atmosphere at a temperature sufficient to initiate cyclization of the nitrile group without causing degradation of the precursor.

In some embodiments, the temperature to which the precursor is heated in a substantially oxygen-free atmosphere may also affect the degree of cyclization of the nitrile group, as it has been found that higher heating temperatures may promote and increase the cyclization of the nitrile group in the precursor.

In some embodiments, it is preferred that the temperature to which the precursor is heated when in a substantially oxygen-free atmosphere is near the degradation temperature of the precursor. High temperatures near the degradation temperature of the precursor can help ensure that a high content of cyclized nitrile groups is obtained in a short period of time.

PAN precursors are generally reported in the literature as having a degradation temperature of from about 300 ℃ to 320 ℃. However, the skilled person will understand that the precursor degradation temperature may differ from the reported literature values as it may depend on the composition of the PAN precursor.

If one of skill in the art wishes to determine the degradation temperature of a given PAN precursor, this can be determined using Differential Scanning Calorimetry (DSC) under a nitrogen atmosphere. Using DSC, a sample of a given precursor can be placed in a nitrogen atmosphere and heated at a rate of 10 ℃/minute. The heat flux (heat flux) was then measured as a function of temperature. Thermal degradation of the precursor can be detected by observing an exothermic transition in the DSC curve. Thus, the temperature corresponding to the peak (or maximum) of the exothermic transition is the degradation temperature of the precursor.

In some embodiments, the precursor is heated in a substantially oxygen-free atmosphere at a temperature no greater than 30 ℃ below the degradation temperature of the precursor. This will be understood to mean that the precursor cannot be heated above the degradation temperature of the precursor and, in addition, cannot be heated below the degradation temperature by more than 30 ℃. Thus, in such embodiments, the PAN precursor may be heated in a substantially oxygen-free atmosphere at a temperature (T) selected to be within a range represented by: (T)_D-30℃)≤T<T_DWherein T is_DIs the degradation temperature (in C.) of the precursor.

In another set of embodiments, the precursor is heated in a substantially oxygen-free atmosphere at a maximum temperature that is at least 5 ℃ below the degradation temperature of the precursor and no more than 30 ℃ below the degradation temperature. This will be understood to mean that the precursor is heated in a substantially oxygen-free atmosphere at a temperature (T) selected to be within the range represented by: (T)_D-30℃)≤T≤(T_D-5 ℃), wherein T_DIs the degradation temperature (in C.) of the precursor.

In one set of embodiments, the precursor fibers are heated in a substantially oxygen-free atmosphere at a maximum temperature of no more than about 400 ℃, preferably no more than about 380 ℃, and more preferably no more than about 320 ℃.

In one set of embodiments, the precursor fibers are heated in a substantially oxygen-free atmosphere at a minimum temperature of not less than about 250 ℃, preferably not less than about 270 ℃, and more preferably not less than about 280 ℃.

In one set of embodiments, the precursor is heated in a substantially oxygen-free atmosphere at a temperature in the range of from about 250 ℃ to 400 ℃, preferably in the range of from about 270 ℃ to 350 ℃, more preferably in the range of from about 280 ℃ to 320 ℃.

During the pre-stabilization step, the precursor may be heated at a substantially constant temperature profile or a variable temperature profile.

At variable temperature profiles, the precursor may be heated at two or more different temperatures. The two or more different temperatures are preferably within the temperature ranges described herein.

In one set of embodiments, the precursor is heated at a substantially constant temperature of about 300 ℃.

In another set of embodiments, the precursor may be initially heated at a selected temperature, and then the temperature may be increased as the pre-stabilization step proceeds. As an example, the PAN precursor may initially be heated at a temperature of about 285 ℃, wherein the temperature is increased to about 295 ℃ during the pre-stabilization step.

Once the one or more temperatures and heating profiles for heating the precursor in the substantially oxygen-free atmosphere are selected, the temperature parameters remain fixed and do not change. For example, in a continuous carbon material (e.g., carbon fiber) manufacturing process incorporating the precursor stabilization process described herein, it may be desirable to maintain a constant and fixed at a selected value for process stability for each temperature parameter selected for pre-stabilization of the precursor as part of the stabilization process, and to enable stable, continuous operation. In some embodiments, it may be desirable for the temperature used in the pre-stabilization step to be maintained within about 2 ℃, preferably within about 1 ℃ of the selected pre-stabilization temperature. It is preferred to limit or avoid undesirable temperature fluctuations during the pre-stabilization step, as these may give rise to undesirable changes in the precursor. For example, the temperature change may cause local hot spots in the precursor, deformation of the precursor, or breakage of the precursor fibers.

Heating the precursor during the pre-stabilization step may occur by passing the precursor through a single temperature zone or multiple temperature zones.

In embodiments where heating the precursor during the pre-stabilization step occurs by passing the precursor through multiple temperature zones, the precursor may be passed through 2,3, 4, or more temperature zones. Each of the zones may have the same temperature. Alternatively, two or more zones may have different temperatures. For example, at least one temperature zone (e.g., a first temperature zone) may be at a first temperature while at least one temperature zone (e.g., a second temperature zone) is at a second temperature different from the first temperature. The precursor may be heated at a variable temperature profile by passing the precursor through a plurality of zones of different temperatures.

In some embodiments, each temperature zone may provide a reaction zone in which a reaction is carried out that promotes the cyclization of a nitrile group in the precursor.

The precursor may be passed through the selected temperature zone in a single pass. For example, when a single temperature zone or multiple temperature zones are used, the precursor fiber may pass through each temperature zone in a single pass.

Alternatively, the precursor may be passed through the selected temperature zone multiple times. Thus, the precursor may be passed through a given temperature zone in multiple passes.

In one set of embodiments, a substantially oxygen-free gas stream may be used to establish a substantially oxygen-free atmosphere. The substantially oxygen-free gas stream may be heated. The heated substantially oxygen-free gas stream may be used to heat the precursor and maintain the precursor at a selected temperature.

In one set of embodiments, the precursor is heated in a substantially oxygen-free atmosphere for a time period ranging from less than one minute (i.e., several seconds) to several minutes.

An advantage of the present invention is that a short period of time for the pre-stabilization step can be achieved by adjustment of the heating temperature and the amount of tension applied to the precursor.

Thus, the temperature for heating the precursor in the substantially oxygen-free atmosphere and the amount of tension applied to the precursor fiber when the precursor fiber is heated can each be selected to promote the formation of at least 10%, at least 15%, or at least 20% of the cyclized nitrile groups in the precursor in a time period of less than 5 minutes, less than 4 minutes, less than 3 minutes, or less than 2 minutes. Thus, a pre-stabilized precursor having a desired amount of cyclized nitrile groups can be formed quickly and in a short period of time.

In a preferred embodiment, the temperature for heating the precursor in the substantially oxygen-free atmosphere and the amount of tension applied to the precursor fiber when the precursor fiber is heated are each selected so as to promote the formation of between 10% -50%, 15% -40%, or 20% -30% of the cyclized nitrile groups in the precursor in a time period of less than 5 minutes, less than 4 minutes, less than 3 minutes, or less than 2 minutes.

The present invention allows the formation of a desired amount of cyclized nitrile groups in a short period of time. Thus, the precursor need only be heated in a substantially oxygen-free atmosphere for a short period of time. Thus, in some embodiments, the precursor may reside in a substantially oxygen-free atmosphere for a period of time of no more than about 5 minutes, no more than about 4 minutes, no more than about 3 minutes, or no more than about 2 minutes.

In some embodiments, heating the precursor at a temperature near the degradation temperature of the precursor may promote rapid formation of a pre-stabilized precursor.

During the pre-stabilization step, a substantially constant amount of tension is also applied to the precursor. One skilled in the relevant art will appreciate that tension is the force applied to the precursor. In the pre-stabilization step, the amount of tension applied to the precursor is maintained at a predetermined and substantially constant value and does not change as the precursor is heated in a substantially oxygen-free atmosphere. As an example, the precursor fiber may be suspended between two tensioning devices (tensioning devices), wherein the tensioning devices operate to ensure that the tension applied to the precursor suspended therebetween is maintained at a substantially constant and predetermined value. Thus, once a certain amount of tension is selected for a given precursor, the tension is maintained so that the precursor can be processed at a substantially constant amount of tension during the pre-stabilization step.

It is desirable that the tension applied to the precursor does not change during pre-stabilization, as changes in tension may indicate or promote process instability. Preferably, there is less than a 5% change in the amount of tension applied to the precursor during the pre-stabilization step.

The amount of tension applied may depend on many factors such as, for example, the temperature and time period for which the precursor is heated in a substantially oxygen-free atmosphere, the composition of the PAN precursor, and the size of the precursor tow. The applied tension may be adapted to enable pre-stabilization process conditions for a particular precursor and/or tow size and/or selected time and temperature to achieve optimal results.

It will also be appreciated that there may be inherent tension effects in the precursor due to physical and/or chemical changes that may occur in the precursor as the pre-stabilization step progresses. However, it is intended that the tension applied to the precursor in accordance with the process of the embodiments described herein will encompass any inherent tension variation that may be produced in the precursor during the pre-stabilization step. The tension applied to the precursor may be such as to accommodate variations in the inherent tension of the precursor due to changes that occur in the precursor during pre-stabilization.

In one set of embodiments, when determining the process conditions (i.e., heating temperature, time period, and tension) to be used for the pre-stabilization step, it may be useful to initially determine a baseline tension sufficient to facilitate the transport of the precursor at a selected rate through the reaction chamber used to perform the pre-stabilization step. The rate at which the precursor is delivered may determine the time for which the precursor stays in the reaction chamber. Once the baseline tension and residence time period in the reaction chamber are determined, the temperature for heating the precursor can then be selected.

The temperature used to heat the precursor in the pre-stabilization step is sufficient to initiate or promote cyclization of a portion of the nitrile groups present in the precursor, but not so high as to cause degradation of the precursor. As discussed above, cyclization of the nitrile group can be visually represented as a change in color of the precursor from white to a color ranging from deep yellow or orange to copper. Thus, the change in color of the precursor provides an indication of when the nitrile group cyclization can be initiated and can be used as a visual cue for selecting the heating temperature.

In practice, to select the heating temperature, the precursor can be heated at a variety of different temperatures while the baseline tension applied to the precursor and the residence time of the precursor in the reaction chamber each remain fixed. Then, the change in color of the precursor is visually determined. The temperature at which the initial color change in the precursor is observed can be considered as the lowest temperature that can be used to pre-stabilize the precursor.

In a preferred embodiment, the PAN precursor is heated at an elevated temperature near the degradation temperature of the precursor while in a substantially oxygen-free atmosphere. It is believed that the use of high heating temperatures near the degradation temperature of the precursor can facilitate the formation of a pre-stabilized precursor having at least 10% cyclized nitrile groups in a time period of less than about 5 minutes, less than about 4 minutes, less than about 3 minutes, or less than about 2 minutes. In one embodiment, the pre-stabilized precursor may have from 20% to 30% cyclized nitrile groups.

In one embodiment, the precursor is heated at a temperature not more than 30 ℃ below the degradation temperature. It has been found that when the PAN precursor is heated at an elevated temperature within 30 ℃ of the degradation temperature of the precursor, a color change may occur in the precursor within a short period of time, e.g. within about 2 minutes. The color change may be visually discernable and may be indicative of a chemical change (such as a cyclization reaction and an aromatization reaction) occurring in the precursor.

Once the heating temperature is determined, the amount of tension applied to the precursor is then adjusted (e.g., increased) from a baseline value until a tension value is determined that promotes a desired level of nitrile group cyclization (% EOR) in the precursor under the selected heating temperature and time conditions. As discussed above,% EOR can be determined by FT-IR spectroscopy.

Once the tension value that gives the desired% EOR in the precursor has been determined, the resulting pre-stabilized precursor can be tested to determine if the precursor has properties within desired parameters such as tensile properties, mass density and appearance. If desired, further adjustments may be made to fine tune the tensioning parameters such that the amount of tension applied to the precursor is sufficient not only to form a pre-stabilized precursor having the desired level of nitrile group cyclization (% EOR), but also to form a pre-stabilized precursor having the desired color, tensile properties, mass density and/or appearance.

In some embodiments, the precursor has the potential to reach the maximum amount of cyclized nitrile groups, and it may be desirable to select the amount of tension applied to the PAN precursor fiber to promote the formation of the maximum amount of cyclized nitrile groups in the pre-stabilized precursor fiber. This tension may be referred to as an "optimized tension" value. Thus, the degree of reaction (% EOR) of the nitrile group achievable in the PAN precursor under a substantially oxygen-free atmosphere is highest at about the optimized strain value.

The optimized tension value may be determined by applying varying amounts of substantially constant tension to the precursor fiber while maintaining the preselected temperature and time conditions constant in a substantially oxygen-free atmosphere. It has been found that as the amount of tension applied to a given precursor fiber increases, the degree of cyclization of nitrile groups (% EOR) as measured by FT-IR spectroscopy increases until a maximum value is reached. The maximum% EOR corresponds to the highest amount of cyclized nitrile groups produced in the precursor fiber under the pre-stabilization conditions employed. After the maximum, the degree or amount of cyclized nitrile group decreases, even if the amount increases with the applied tension. Thus, a "bell-shaped"% EOR versus tension curve may be formed. The bell-shaped curve will typically contain a peak% EOR, which will correspond to the maximum% EOR achievable for the given precursor. Thus, the strain value that provides the highest degree of nitrile group cyclization (i.e., maximum% EOR) at the preselected temperature and time parameters is the optimized strain for the PAN precursor.

In some embodiments, it may be desirable for the pre-stabilized precursor to have a maximum amount of cyclized nitrile groups to enable formation of the stabilized precursor with improved efficiency.

In some embodiments of the invention, the precursor has the potential to achieve a maximum amount of cyclization of the nitrile group, and the amount of tension applied to the precursor is selected to promote the maximum cyclization of the nitrile group in the precursor. In such embodiments, when the precursor is heated at a selected temperature and for a selected period of time in a substantially oxygen-free atmosphere, an optimized amount of tension can therefore be applied to the precursor in order to form a pre-stabilized precursor having a maximum amount of cyclized nitrile groups. The optimized strain will produce at least 10% cyclized nitrile groups in the precursor, and may and preferably will produce more than 10% cyclized nitrile groups in the precursor.

It will be appreciated that due to the slightly different polymer compositions of PAN precursors from different commercial suppliers, the different maximum% EOR achievable for PAN precursors and the optimized strain that can facilitate maximized nitrile group cyclization may be different for different precursors. For example, PAN precursors may differ in a range of parameters such as composition and tow size. Thus, it will be appreciated that the optimum strain and maximum amount of cyclized nitrile group achievable in the precursor may vary with different precursor starting materials. For example, for some precursor starting materials, a potential maximum of 40% cyclized nitrile groups may be achieved, while for other precursor starting materials, a maximum of 20% cyclized nitrile groups may only be possible.

In some embodiments, there may be an acceptable window of operation for the tonicity parameter such that a pre-stabilized precursor may be formed having an amount of nitrile groups that is greater than 10% but less than the maximum amount of cyclized nitrile groups achievable by the precursor. That is, it may be that the pre-stabilized precursor may have an intermediate amount of cyclized nitrile groups that varies from and is less than the maximum% EOR, but is still greater than 10%.

In some embodiments, the pre-stabilized precursor may have an optimal amount of cyclized nitrile groups, wherein the optimal amount includes the maximum amount of cyclized nitrile groups (maximum% EOR), and acceptable variations thereof. Thus, an "optimal amount" may include the maximum% EOR achievable for a given precursor at an optimized tension, as well as an acceptable sub-maximum of the% EOR achieved at an applied tension amount above or below the optimized tension. In the context of the% EOR versus tonicity curve, the "optimal amount" of cyclized nitrile group is the amount within an acceptable operating window defined by the region surrounding the% EOR versus the maximum% EOR in the tonicity curve and encompassing acceptable values of% EOR below the maximum% EOR.

While at less than a maximum, the optimal amount of cyclized nitrile group may still be beneficial in facilitating efficient formation of the pre-stabilized and stabilized precursor.

The amount of change from the maximum% EOR that qualifies as the optimum amount of nitrile groups for cyclization and is considered acceptable for efficient precursor processing may depend on the values of the precursor and the maximum% EOR. The skilled person will appreciate that where higher maximum% EOR values can be reached in the precursor, larger variations from maximum% EOR may be acceptable, whereas smaller variations from maximum% EOR may only be acceptable when only smaller maximum% EOR values are achievable.

In some embodiments, for precursors having the potential to achieve the maximum amount of cyclized nitrile groups, the amount of strain applied to the precursor is selected to promote up to 80% less cyclization than the maximum achievable nitrile group in the pre-stabilized precursor. In some embodiments, the amount of tension applied to the precursor can be selected to promote up to 70% less, up to 60% less, up to 50% less, up to 40% less, up to 30% less, or up to 20% less cyclization than the maximum achievable nitrile group cyclization in the pre-stabilized precursor. Each of the above-mentioned ranges may independently represent a window within which an optimal amount of cyclized nitrile groups may be formed in a given precursor.

In one illustrative example, where the maximum amount of cyclized nitrile groups that can be achieved in the precursor is 50%, the tension applied to the precursor can be selected so as to form a pre-stabilized precursor having an amount of cyclized nitrile groups in the range of from 10% to 50%. Thus, in this example, there may be an acceptable operating range of% EOR of up to 40%. Furthermore, in this example, the amount of 10% represents the minimum amount of cyclized nitrile groups acceptable for the pre-stabilized precursor according to the invention. This 10% value also represents an amount of about 80% of the maximum achievable cyclization of the nitrile group (i.e. 80% of 50%). Thus, the amount of nitrile group representing the optimum amount of cyclization may be selected from amounts in the range of from 10% to 50%, and in some embodiments, the strain may be selected to promote the amount of cyclized nitrile group in this% EOR range.

In another illustrative example, where the maximum amount of cyclized nitrile groups that can be achieved in the precursor is 30%, the tension applied to the precursor can be selected so as to form a pre-stabilized precursor having an amount of cyclized nitrile groups in the range of from 10% to 30%. Thus, in this example, there may be an acceptable operating range in% EOR up to 20%. Thus, the minimum value of 10% of cyclized nitrile groups represents an amount of about 67% of the maximum achievable cyclization of the nitrile groups (i.e., 67% of 30%). Similar to the illustrative examples above, the amount of nitrile group representing the optimal amount of cyclization can thus be selected from amounts ranging from 10% to 30%, and in some embodiments, the strain that promotes the amount of cyclized nitrile group in this% EOR range can be selected.

In yet another illustrative example, where the maximum amount of cyclized nitrile groups that can be achieved in the precursor is 20%, 80% less than the maximum achievable cyclization of nitrile groups represents 4% cyclized nitrile groups. However, it will be appreciated that the value of 4% is below the minimum threshold of at least 10% of cyclized nitrile groups required for a pre-stabilized precursor according to the invention. In such a case, the acceptable operating window will therefore be limited by a lower threshold of 10% cyclized nitrile groups, so that the tension applied to the precursor can be selected only from the tensions forming an amount of cyclized nitrile groups in the range between from 10% to 20%. Thus, in this example, an operating window providing only up to 50% (i.e. 50% of 20%) of the maximum achievable cyclization of the nitrile group is acceptable. Thus, the amount of cyclized nitrile group in the range of from 10% to 20% can represent the optimal amount of cyclized nitrile group, and in some embodiments, the strain can be selected to promote the amount of cyclized nitrile group in this% EOR range.

In some embodiments, a pre-stabilized precursor may have at least 15% or at least 20% cyclized nitrile groups as a lower threshold (or minimum) amount of cyclized nitrile groups. In such embodiments, the acceptable amount of variation from the maximum% EOR may be within a smaller window. For example, where the maximum amount of cyclized nitrile groups that can be achieved in the precursor is 50% and a minimum (or lower threshold) of 15% cyclization of nitrile groups is required in the formed pre-stabilized precursor, the tension applied to the precursor can be selected so as to form an amount of cyclized nitrile groups in the range of from 15% to 50%. Thus, in this example, there may be an acceptable operating range in the% EOR up to 35%. Thus, the minimum degree of 15% nitrile cyclization represents an amount of about 70% (i.e., 70% of 50%) of the maximum nitrile group cyclization.

The maximum amount and optimum amount of nitrile groups cyclized can each be determined using fourier transform infrared (FT-IR) spectroscopy as described herein.

In embodiments where more than 10% of the desired amount of cyclized nitrile groups in the pre-stabilized precursor, but less than the potentially maximum amount of cyclized nitrile groups available in the precursor, the amount of tension applied to the precursor can be different than the optimized tension value of the precursor in order to facilitate formation of the desired amount of cyclized groups. The change from the optimized strain can be a strain value that is higher or lower than the optimized strain value that promotes the maximum nitrile group cyclization.

In one set of embodiments, the precursor, when heated at a selected temperature and for a selected period of time in a substantially oxygen-free atmosphere, can be subjected to a strain in an amount up to 20% different from the optimized strain to form a pre-stabilized precursor having at least 10% cyclized nitrile groups. In other embodiments, the tension may be applied to the precursor in an amount up to 15% or up to 10% different from the optimized tension to form a pre-stabilized precursor having at least 10% cyclized nitrile groups.

One or more embodiments of the precursor stabilization processes described herein can further include the step of determining a tension parameter of the precursor prior to forming the pre-stabilized precursor, wherein determining the tension parameter of the precursor comprises:

(e) determining from the calculated trend an amount of strain providing at least 10% nitrile group cyclization and maximum nitrile group cyclization; and

(f) the amount of tension that causes at least 10% of the nitrile groups to cyclize is selected so as to pre-stabilize the precursor.

The determination of the tension parameters is desirably performed on the precursor prior to the stabilization process of the present invention with respect to the precursor. Suitably, the determination of the tension parameter will be made prior to formation of the pre-stabilised precursor from the precursor.

Determination of the tonicity parameter will facilitate the determination and selection of an appropriate amount of tonicity to facilitate the desired degree of cyclization of the nitrile group in a given precursor under selected conditions of temperature and time period. This may enable the formation of a pre-stabilized precursor having a desired amount of cyclized nitrile groups when the precursor is heated in a substantially oxygen-free atmosphere at a selected temperature and time period as part of the stabilization process.

The determination of the tension parameter may facilitate the determination of an amount of tension that may facilitate the formation of the following in the precursor when the precursor is heated in a substantially oxygen-free atmosphere at the selected temperature and time parameters: (i) at least 10% of the cyclized nitrile groups in a given precursor, (ii) the maximum achievable amount of cyclized nitrile groups in the precursor, and (iii) an intermediate amount of cyclized nitrile groups that occurs between 10% and the maximum achievable amount. Thus, the above tonicity parameter determining step may be used to facilitate screening of the amount of tonicity that will achieve the desired degree of cyclization of the nitrile group (% EOR) in the pre-stabilized precursor produced from the precursor to be evaluated.

The determining of the tension parameter of the precursor includes applying a series of different substantially constant amounts of tension to the precursor while the precursor is heated at a selected temperature and for a selected period of time in a substantially oxygen-free atmosphere. Thus, during this evaluation, the temperature and time period for heating the precursor each remain fixed at selected values.

The determination of the tension parameter includes applying different amounts of substantially constant tension to the precursor fiber while selected conditions of temperature and time for heating the precursor in a substantially oxygen-free atmosphere are each maintained at a selected value. In practice, it is useful to apply an initial tension to the precursor, which may be a baseline tension. As discussed above, the baseline tension is a baseline tension sufficient to facilitate the delivery of the precursor through the pre-stabilization reactor. Then, the amount of tension applied to the precursor may be incrementally increased from the initial tension value. The amount of cyclized nitrile group (% EOR) formed in the precursor when a series of different substantially constant amounts of tension were applied to the precursor was then determined by FT-IR spectroscopy.

Once data are obtained regarding the amount of cyclized nitrile group (% EOR) formed at different amounts of applied tonicity, the trend of the degree of cyclization of the nitrile group (% EOR) with respect to tonicity can then be calculated. In some embodiments, calculation of the trend of the degree of cyclization of the nitrile group (% EOR) versus tonicity may comprise generating a graph illustrating the% EOR versus tonicity curve.

From the calculated trend of the degree of cyclization of the nitrile group (% EOR) versus the tonicity, the amount of tonicity can then be determined which promotes in the precursor (i) at least 10% cyclization of the nitrile group, (ii) maximum cyclization of the nitrile group, and (iii) an intermediate amount of cyclization of the nitrile between 10% and the maximum achievable amount. For example, from the calculated trend, the amount of strain that promotes the formation of from 20% to 30% of cyclized nitrile groups in the precursor can be determined.

Once the amount of strain that produces or promotes the formation of the desired, selected% EOR in the precursor at the selected temperature and time period has been determined from the calculated trend, the amount of strain may be selected for pre-stabilization of the precursor.

Typically, the amount of tension that promotes at least 10% cyclization of the nitrile groups is selected to pre-stabilize the precursor in the pre-stabilization step described herein.

In some embodiments, the amount of tension that promotes from 10% to 50%, from 15% to 45%, or from 20% to 30% cyclization of the nitrile group is selected to pre-stabilize the precursor in the pre-stabilization step described herein.

In yet other embodiments, an amount of strain that is up to 80%, up to 70%, up to 60%, up to 50%, up to 40%, up to 30%, or up to 20% less than the maximum nitrile group cyclization achievable in the precursor is selected to pre-stabilize the precursor in the pre-stabilization step described herein.

In other embodiments, the amount of strain that promotes maximum nitrile cyclization is selected to pre-stabilize the precursor in the pre-stabilization step described herein.

The skilled person will understand that in addition to the selected tension parameters for the pre-stabilization step (which have been determined according to the above steps), the temperature and time period used in determining the tension parameters will also be used for pre-stabilization of the precursor. This is because the desired strain parameters for properly forming a pre-stabilized precursor with the requisite amount of cyclized nitrile groups can vary if different temperature and/or time period conditions are used for pre-stabilization of a given precursor.

In one set of embodiments, the pre-stabilization of the PAN precursor comprises heating a precursor comprising polyacrylonitrile in a substantially oxygen-free atmosphere for a period of time not exceeding 5 minutes while applying a substantially constant amount of tension to the precursor, the temperature at which the precursor is heated in the substantially oxygen-free atmosphere and the amount of tension applied to the precursor being sufficient to form a pre-stabilized precursor comprising at least 10% cyclized nitrile groups as determined by fourier transform infrared (FT-IR) spectroscopy.

As discussed above, the tension applied to the precursor can control the degree of cyclization of the nitrile groups in the precursor, and thus enable the desired amount of cyclized nitrile groups to be achieved. In some embodiments of the pre-stabilization processes described herein, the tension applied to the precursor is sufficient to form a pre-stabilized precursor having at least 15% and preferably from between 20% and 30% cyclized nitrile groups as determined by FT-IR spectroscopy.

In one set of embodiments, during the pre-stabilization step, the precursor is heated at a predetermined temperature in a substantially oxygen-free atmosphere for a predetermined period of time while applying a substantially constant amount of tension to the precursor sufficient to form a pre-stabilized precursor having at least 10% cyclized nitrile groups as determined by FT-IR spectroscopy. The skilled person will understand that a value of 10% represents the minimum amount of cyclized nitrile groups in the pre-stabilized precursor, and that higher amounts of cyclized nitrile groups may be formed in the pre-stabilized precursor. For example, the pre-stabilized precursor may have from 20% to 30% cyclized nitrile groups.

In a particular set of embodiments, the pre-stabilization of the PAN precursor comprises heating a precursor comprising polyacrylonitrile in a substantially oxygen-free atmosphere at a temperature in the range of from about 250 ℃ to 400 ℃ for a period of time of not more than 5 minutes while applying a substantially constant amount of tension to the precursor, the amount of tension being sufficient to form a pre-stabilized precursor having at least 10% cyclized nitrile groups as determined by fourier transform infrared (FT-IR) spectroscopy.

In some embodiments, the pre-stabilized precursor may have from 10% to 50%, from 15% to 40%, or from 20% to 30% cyclized nitrile groups as determined by FT-IR spectroscopy.

The desired amount of cyclized nitrile groups is formed during the residence time of the precursor in the substantially oxygen-free atmosphere. Thus, a desired amount of cyclized nitrile group can be formed within a time period selected from less than 5 minutes, less than 4 minutes, less than 3 minutes, or less than 2 minutes.

In some embodiments, the precursor comprising polyacrylonitrile is heated in a substantially oxygen-free atmosphere for a period of time of no more than 4 minutes, no more than 3 minutes, or no more than 2 minutes.

In some embodiments, the polyacrylonitrile-containing precursor is heated at a temperature in the range of from about 280 ℃ to 320 ℃ in a substantially oxygen-free atmosphere.

In some embodiments, the amount of tension applied to the precursor is sufficient to form a pre-stabilized precursor having at least 15% or at least 20% cyclized nitrile groups. The degree of cyclization of the nitrile group is determined by fourier transform infrared (FT-IR) spectroscopy, as described herein. It has been found that in some embodiments, an insufficient degree of cyclization can occur if insufficient tension is applied to the precursor.

In some embodiments, the amount of tension applied to the precursor is sufficient to form a pre-stabilized precursor having from between about 10% to about 50%, preferably from about 10% to about 45%, and most preferably from about 20% to about 30% cyclized nitrile groups as determined by FT-IR spectroscopy.

The optimum amount of cyclized nitrile group in the pre-stabilized precursor can fall within the above ranges. In some embodiments, the maximum amount of cyclized nitrile group (maximum% EOR) achievable in the pre-stabilized precursor can be within the ranges above.

For a selected PAN precursor fiber and selected heating time and temperature conditions for the pre-stabilization step, the amount of tension applied to the precursor fiber should be such that the precursor fiber is not in a relaxed state. For practical considerations, the tension applied to the precursor will be sufficient to facilitate the transport of the fibers through the reaction chamber used to perform the pre-stabilization step, while also avoiding contact with the inner surfaces of the chamber. However, the applied tension should also not be so high that the precursor fibers break under the applied tension.

In one set of embodiments, the amount of tension applied to the PAN precursor ranges from about 50cN to about 50,000cN, depending on tow size. In some embodiments, the amount of tension applied to the PAN precursor may range from about 50cN to about 10,000 cN. For example, in some embodiments, a tension of up to 6,000cN may be applied. In some embodiments, a tension of up to 4,000cN may be applied.

In some embodiments, the tension applied to the PAN precursor is not sufficient to change the size (e.g., shape or length) of the precursor to a significant extent. For example, at least one dimension of the precursor does not change by more than 10% after tension is applied to the PAN precursor.

Once a suitable tension to promote cyclization of the desired amount of nitrile groups in a given precursor is selected, the tension applied to the precursor remains substantially constant and fixed. A controller (controls) may be utilized to ensure that the tension remains within acceptable limits from selected values so that the precursor is processed at a substantially constant tension. This may be important to ensure that tension is maintained to ensure stable precursor processing, which may facilitate continuous operation of the precursor stabilization process and ensure consistent quality in the pre-stabilized precursor, and subsequently also in the carbon fiber.

In one set of embodiments, the amount of tension applied to the PAN precursor during the pre-stabilization step is selected to maximize the degree of cyclization of the nitrile groups in the precursor.

In one set of embodiments, during the pre-stabilization step, the precursor is heated at a predetermined temperature in a substantially oxygen-free atmosphere for a predetermined period of time while applying a substantially constant amount of tension to the precursor sufficient to form a pre-stabilized precursor having a maximum degree of cyclization of nitrile groups (maximum% EOR) as determined by FT-IR spectroscopy.

In particular embodiments, the predetermined period of time that the precursor is heated to achieve maximum nitrile cyclization (maximum% EOR) may be selected from no more than about 5 minutes, no more than about 4 minutes, no more than about 3 minutes, or no more than about 2 minutes.

In particular embodiments, the predetermined temperature at which the precursor is heated to achieve maximum nitrile cyclization (maximum% EOR) may range from about 250 ℃ to 400 ℃ or from about 280 ℃ to 320 ℃.

In particular embodiments, the tension applied to the precursor to achieve maximum nitrile cyclization (maximum% EOR) may be in the range of from about 50cN to about 50,000cN, or in the range of from about 50cN to about 10,000 cN.

The amount of tension applied during pre-stabilization may promote the rapid formation of the necessary amount of cyclized nitrile groups in the PAN precursor.

In some embodiments, it may be beneficial to apply an optimized tension value to the precursors of an economical process for producing carbon materials, such as carbon fibers.

In some embodiments, the tension applied to the precursor during the pre-stabilization step is such that the elongation spread (standard deviation) as determined by a Favimat (monofilament tester) is as low as possible. A small standard deviation and thus a small elongation spread can help determine whether the precursor fiber is uniformly processed. In a preferred version, the applied tension is such that the elongation spread of the pre-stabilization step is as close as possible to the elongation spread of the untreated (raw) PAN precursor.

In some embodiments, the tension applied to the precursor is insufficient to cause stretching of the precursor, which may result in an increase in the length of the precursor after the pre-stabilization step.

In certain embodiments, it may be preferred to control the amount of tension applied to the precursor so as to form a pre-stabilized precursor having a stretch ratio (stretch ratio) of 0% or less. The 0% stretch ratio may be achieved by operating the process equipment in a manner that ensures that the precursor is not stretched.

In another set of embodiments, during the pre-stabilization step, the precursor is heated at a predetermined temperature in a substantially oxygen-free atmosphere for a predetermined period of time while applying a substantially constant amount of tension to the precursor in an amount sufficient to form a pre-stabilized precursor comprising an optimized amount of cyclized nitrile groups as determined by FT-IR spectroscopy.

In a particular embodiment, the pre-stabilization of the PAN precursor comprises heating the precursor comprising polyacrylonitrile in a substantially oxygen-free atmosphere at a temperature in the range of from about 250 ℃ to 400 ℃ for a period of time not exceeding 5 minutes while applying a substantially constant amount of tension to the precursor, the amount of tension being selected to form a pre-stabilized precursor comprising an optimized amount of cyclized nitrile groups as determined by fourier transform infrared (FT-IR) spectroscopy.

As discussed herein, the optimal amount of cyclized nitrile groups can be an amount up to 80%, up to 70%, up to 60%, up to 50%, up to 40%, up to 30%, or up to 20% less than the maximum amount of cyclized nitrile groups achievable in the precursor.

In particular embodiments, the predetermined period of time for which the precursor is heated to achieve an optimal amount of cyclization of the nitrile group may be selected from no more than about 5 minutes, no more than about 4 minutes, no more than about 3 minutes, or no more than about 2 minutes.

In particular embodiments, the predetermined temperature at which the precursor is heated to obtain an optimized amount of cyclization of the nitrile group may range from about 250 ℃ to 400 ℃ or from about 280 ℃ to 320 ℃.

In particular embodiments, the tension applied to the precursor to achieve an optimal amount of cyclization of the nitrile groups may be in the range of from about 50cN to about 50,000cN, or in the range of from about 50cN to about 10,000 cN.

The pre-stabilization treatment of PAN precursors according to the processes described herein can be performed in a variety of different devices.

In one set of embodiments, the pre-stabilization step is carried out in a reactor suitable for heating the precursors contained therein in a substantially oxygen-free atmosphere. PAN precursors are transported through the reactor during the pre-stabilization step. Reactors suitable for pre-stabilizing precursors may also be referred to herein as "pre-stabilization reactors".

The pre-stabilization reactor may include a reaction chamber adapted to pre-stabilize the precursor in a substantially oxygen-free atmosphere as the precursor passes through the reaction chamber under a predetermined amount of tension; an inlet for allowing a precursor to enter the reaction chamber; an outlet for allowing the precursor to exit the reaction chamber; and a gas delivery system for delivering a substantially oxygen-free gas stream to the reaction chamber. The precursor fiber may be passed through the reaction chamber in a single pass or in multiple passes. In some embodiments, the pre-stabilization reactor may further comprise a cooling zone adapted to remove heat from the pre-stabilized precursor before the pre-stabilized precursor exits the reactor.

An example of a pre-stabilization reactor may be a furnace or oven adapted to contain a substantially oxygen-free atmosphere.

Another example of a reactor that can be used to pre-stabilise the precursor is described in australian provisional patent application no 2016904219 and co-pending international patent application claiming priority from australian provisional patent application no 2016904219.

The pre-stabilization reactor may comprise a single reaction chamber or a plurality of reaction chambers. Where the reactor comprises a plurality of reaction chambers, each chamber may be at the same temperature, or two or more chambers may be at different temperatures. Each reaction chamber may provide a temperature zone in which the PAN precursor is heated.

In a preferred embodiment, the pre-stabilization reactor may comprise a single reaction chamber. A single reaction chamber may be adapted to provide multiple temperature zones for pre-stabilizing the precursor.

Where the pre-stabilization reactor comprises a plurality of temperature zones (which may be in a single reaction chamber or in a plurality of reaction chambers), each temperature zone is preferably within the pre-stabilization temperature range described herein.

In the case where the precursor is passed through the pre-stabilisation reactor in multiple passes, the pre-stabilisation step may be carried out in multiple stages. Each pass may represent a stage of the pre-stabilization step. The pre-stabilization step may be interrupted between each stage.

In some embodiments, where the pre-stabilization is performed in multiple stages, it is contemplated that the precursor may be exposed to the atmosphere briefly between stages. The atmosphere may be a substantially oxygen-free atmosphere such as a nitrogen atmosphere, or alternatively, it may be an oxygen-containing atmosphere such as ambient air.

When the precursor is exposed to an oxygen-containing atmosphere between stages of the pre-stabilization step, it is desirable that such exposure be as short as possible (e.g., on the order of seconds) so that no substantial chemical or visual change (e.g., color change) occurs or is discerned in the precursor between the pre-stabilization stages.

It is also contemplated that the precursor may be cooled between stages of the pre-stabilization step. This may be desirable in order to limit the reaction of the precursor with oxygen in the surrounding atmosphere if the precursor is briefly exposed to an oxygen-containing atmosphere, such as ambient air, as the precursor is passed through the pre-stabilization reactor in multiple passes.

A substantially oxygen-free gas stream may be used to establish a substantially oxygen-free atmosphere in the reaction chamber of the pre-stabilization reactor. In embodiments, the substantially oxygen-free gas flow may be sufficient to inhibit oxygen from entering the reaction chamber. The substantially oxygen-free gas stream may also help dissipate exothermic energy released when nitrile groups in the PAN precursor undergo cyclization during the pre-stabilization step.

In a preferred embodiment, the substantially oxygen-free gas is an inert gas. The substantially oxygen-free gas may include nitrogen or a noble gas such as argon, helium, neon, krypton, xenon, and radium, or mixtures thereof.

It is preferred that the substantially oxygen-free gas be as dry as possible and substantially free of water.

In some embodiments, the substantially oxygen-free gas may be a heated gas. The heated gas may be used to establish a substantially oxygen-free atmosphere at a desired temperature in the pre-stabilization reactor. Thus, when in the pre-stabilization reactor, the heated gas may facilitate heating of the PAN precursor at a selected temperature. The use of preheated gas may advantageously help to reduce the overall energy consumption of the precursor stabilization process and the carbon fiber manufacturing process described herein, as energy would not be required to bring the cooling gas in the reactor to the desired temperature. Furthermore, when using heated gas, there may also be a lower gas consumption due to gas expansion.

Those skilled in the art will appreciate that the pre-stabilization reactor will have a defined length, which may depend in part on the number and configuration of reaction chambers in the reactor. The precursor may be passed through each reaction chamber in the reactor in a single pass or in multiple passes at a predetermined velocity. The length of the reactor, the precursor flow path through each reaction chamber in the reactor, and the rate at which the precursor is delivered through the reaction chambers in the reactor can each affect the total residence time of the precursor in the reactor. Further, the residence time may determine the time period for which the pre-stabilization step is performed.

Furthermore, the residence time of the PAN precursor in the reaction chamber may be influenced by the temperature within a given reaction chamber, and vice versa. For example, in embodiments where a higher temperature is used for pre-stabilization, it may be desirable to shorten the residence time in the reaction chamber as compared to embodiments where a lower temperature is used.

In one set of embodiments, the residence time of the precursor in the reactor is no more than about 5 minutes, no more than about 4 minutes, no more than about 3 minutes, or no more than about 2 minutes.

For a given reactor, the temperature of one or more reaction chambers in the reactor, as well as the rate at which precursors are delivered through each chamber and the flow path of precursors through each chamber, may be adjusted in order to achieve a desired residence time.

In some embodiments, the rate at which the precursor is delivered through the pre-stabilization reactor is selected to match the line speed used in the carbon fiber production line. This may allow the pre-stabilization step to be incorporated as a step in existing carbon fiber manufacturing processes. In particular embodiments, the precursor may be conveyed through the pre-stabilization reactor at a speed in a range from about 10 meters per hour (m/h) to 1,000 meters per hour (m/h).

In one form that is capable of undergoing the application of a desired amount of tension, high speeds may be preferred to facilitate rapid processing of the precursor. For example, it may be preferred to have as short a residence time of the precursor as possible in the pre-stabilization reactor.

Once the pre-stabilization period (e.g., residence time in the pre-stabilization reactor) has been selected, the temperature at which the precursor is heated during the pre-stabilization step can then be selected to allow the pre-stabilization step to complete within the selected period of time. Examples of procedures for determining the heating temperature are described above.

In some particular embodiments, during the pre-stabilization step, the precursor is heated in a substantially oxygen-free atmosphere at a temperature sufficient to initiate cyclization of nitrile groups in the precursor without degradation of the precursor. In a preferred embodiment, the temperature to which the precursor is heated while in a substantially oxygen-free atmosphere is sufficient to promote cyclization of at least 10% of the nitrile groups.

In one set of embodiments, the precursor is heated in a substantially oxygen-free atmosphere at a temperature in the range of from about 250 ℃ to 400 ℃ or from about 280 ℃ to 320 ℃. In some embodiments, during the pre-stabilization step, the precursor is heated in a substantially oxygen-free atmosphere at a temperature within a range selected from the group consisting of: 250 ℃ to 400 ℃, from about 260 ℃ to 380 ℃, from about 280 ℃ to 320 ℃ and from about 290 ℃ to 310 ℃. Temperature heating within such a range may occur for a period of time selected from the group consisting of: no more than about 5 minutes, no more than about 4 minutes, no more than about 3 minutes, or no more than about 2 minutes.

The above-mentioned temperature may represent the ambient temperature within the or each reaction chamber of the pre-stabilisation reactor. The ambient temperature may be measured by a thermocouple or other suitable temperature measuring device. The ambient temperature within each reaction chamber of the pre-stabilization reactor is preferably kept substantially constant during the pre-stabilization step.

The pre-stabilization reactor may include one or more heating elements to facilitate heating the precursor at a desired temperature. In some embodiments, the heating element may heat a reaction chamber adapted to pre-stabilize the precursor. The heating element may heat the substantially oxygen-free gas stream delivered to the reaction chamber.

In some embodiments, the temperature within each temperature zone used to pre-stabilize the precursor is preferably maintained within 3 ℃ of the selected temperature value. One or more reaction chambers in the pre-stabilization reactor may provide one or more temperature zones for pre-stabilizing the precursor. In one set of embodiments, where a heated substantially oxygen-free gas stream is used to provide a substantially oxygen-free atmosphere in the pre-stabilization reactor, the temperature of the heated gas as it enters the reactor can be controlled such that the temperature of any one temperature zone does not vary by more than ± 2 ℃, preferably by more than ± 1 ℃, from the desired temperature, so as to enable the reactor temperature to be maintained.

During pre-stabilization, exothermic energy is released as the nitrile group in the PAN precursor undergoes cyclization. If left unmanaged, the amount of exothermic energy released can cause the temperature of the precursor to increase significantly, damaging the precursor and posing a fire risk. To avoid exothermic runaway, the temperature and flow rate of the heated substantially oxygen-free gas may be selected to maintain the temperature of the precursor within acceptable limits. The skilled person will appreciate that when the exothermic energy released causes the precursor to reach a temperature above the ambient temperature of the reactor, then the substantially oxygen-free gas stream may act to cool and control the temperature of the precursor to the desired reactor temperature.

In some embodiments, the gas flow will be such that the temperature measured adjacent the precursor is within 40 ℃ of the temperature of the substantially oxygen-free gas, preferably within 30 ℃ of the temperature of the gas. As used herein, "adjacent to the precursor" means within 10mm of the precursor, preferably within 3mm of the precursor, more preferably within 1mm of the precursor. In some embodiments, the gas flow rate may be such that the actual precursor temperature is within 50 ℃ of the temperature of the gas, preferably within 40 ℃ of the temperature of the gas, more preferably within 30 ℃ of the temperature of the gas.

The flow rate of the gas may be sufficiently high that there is a locally turbulent gas flow around the precursor. This local turbulence in the vicinity of the precursor may induce some fiber agitation and shaking, which promotes efficient removal of reaction by-products, as well as helps in the management of the exothermic behavior of the precursor. Agitation of the precursor fibers in the gas stream may facilitate heat transfer from the precursor to the gas stream to ensure that the temperature of the fibers remains within acceptable limits. However, the flow of gas will be controlled so that it is not too high, as this may cause excessive agitation of the precursor, leading to damage (including breakage) of the precursor.

As discussed above, a substantially constant amount of tension is also applied to the precursor during the pre-stabilization step. The desired amount of tension may be applied by tensioning devices located upstream and downstream of each reaction chamber used to pre-stabilize the precursor. The precursor is suspended between tensioning means adapted to pass the precursor through each reaction chamber under a predetermined amount of tension.

In some embodiments, the tensioning device is a material handling device (such as a material handling device known in the art) and is a separate component from the pre-stabilization reaction chamber. Examples of material handling devices include drive rollers.

In some embodiments, the reactor will include one or more of the tensioning devices. In embodiments where the pre-stabilization reactor comprises two or more reaction chambers, a tensioning device may be provided upstream and downstream of each reaction chamber, such that the precursor is delivered via the tensioning device as it passes from one reaction chamber to the next.

The tensioning device may be controlled by a tension controller to enable a substantially constant amount of tension to be applied to the PAN precursor. The amount of applied tension can be monitored by using a tensiometer or an electrical pressure measuring element such as a piezoelectric load cell. The tensioning device may be controlled to maintain the tension applied to the precursor at a substantially constant value during pre-stabilization. Fluctuations in tension may indicate process instability, which may require adjustment of pre-stabilization process parameters such as temperature.

For example, the drive roller system may allow the precursor fiber to be conveyed through a pre-stabilization step. The speed at which the drive roller system operates can help set the tension used in the pre-stabilization step. The load cell may be used to help monitor any fluctuations in the tension applied to the precursor and provide feedback to help control the tension. The load cells may also be programmed to control tension by an automated system. Preferably, the drive roller system for delivering the precursor is capable of maintaining the tension within 5% of a selected value.

The tension of the precursor can be affected by a number of factors including the relative temperature and humidity of the precursor prior to entering the reactor, the catenary effect (catenariefect) which is affected by the distance between material handling devices (e.g., rolls), the degree of shrinkage experienced by the precursor due to chemical changes occurring in the precursor, and other inherent material property changes that occur when the precursor is pre-stabilized.

In some embodiments, in order to apply a substantially constant amount of tension to the precursor, the draft ratio (draw ratio) applied by the tensioning device will be adjusted as needed. Thus, in practice, for the same precursor at a given temperature and residence time in the pre-stabilization reactor, the draw down ratio applied by the tensioning device may be varied or adjusted to take into account factors affecting the tension of the precursor in order to ensure that a desired, predetermined, substantially constant tension is applied to the precursor. For example, for reactors having a relatively shorter distance between the rollers, a different draft ratio may be applied than for reactors having a longer length, such that the same desired predetermined substantially constant amount of tension may be applied to the precursor in each reactor.

The draft ratio is determined by comparing the conveying speed of the tensioner upstream (i.e. at the inlet side) of the pre-stabilization reactor with the conveying speed of the tensioner downstream (i.e. at the outlet side). When the downstream conveying speed is higher than the upstream speed, the draft ratio is positive and an elongation load is applied to the precursor to increase the applied tension. Conversely, where the upstream speed is higher than the downstream speed, the draft ratio is negative and a compressive load is applied to the precursor to reduce the applied tension. In some embodiments, the degree of shrinkage and other inherent material property changes may be such that a negative draft ratio is used in order to apply a desired predetermined substantially constant tension to the precursor. In other embodiments, a positive draw ratio may be used.

In some other embodiments, the conveying speed is selected such that a 0% draw ratio is used. Thus, in some embodiments, the tensioning devices located upstream and downstream of the pre-stabilization reactor may be operated in a manner that ensures that a desired amount of tension may be applied to the suspended precursor fibers without stretching the precursor fibers. For example, the drive rollers in the tensioning devices located upstream and downstream of the pre-stabilization reaction chamber may operate at the same rotational speed to ensure that the precursor fiber suspended therebetween is not stretched as it travels through the reactor.

In other embodiments, the tensioning devices located upstream and downstream of the pre-stabilization reactor may be operated in a manner that ensures that a desired amount of tension can be applied to the suspended precursor fibers without stretching the precursor fibers. For example, the drive rollers in the tensioning devices located upstream and downstream of the pre-stabilization reaction chamber may operate at the same rotational speed to ensure that the precursor fiber suspended therebetween is not stretched as it travels through the reactor.

If desired, the pre-stabilized precursor may optionally be collected prior to exposure to an oxygen-containing atmosphere. For example, the pre-stabilized precursor may be collected on one or more reels.

However, it is believed that the pre-stabilized precursor is activated for the oxidation treatment step at least in part due to partial cyclization of the PAN precursor during pre-stabilization. Due to this activation, the pre-stabilized precursor may be chemically unstable and susceptible to further reaction when in an oxygen-containing environment (such as air). For example, it is believed that dihydropyridine structures that may be produced in an inert atmosphere may tend to undergo free radical autoxidation reactions when exposed to oxygen. Because of this instability, it may be advantageous to expose the pre-stabilized precursor to an oxygen-containing atmosphere immediately or shortly after its formation rather than to store the pre-stabilized precursor. If it is desired to store the pre-stabilized precursor, it may be beneficial to effect storage in a substantially oxygen-free atmosphere, such as an atmosphere comprising an inert gas.

The pre-stabilized precursor obtained from the pre-stabilization step is considered to be more thermally stable than the original PAN precursor and was found to have a lower exothermicity as determined by Differential Scanning Calorimetry (DSC). It is believed that the reduction in the exothermic behavior of the pre-stabilized precursor is due, at least in part, to the presence of a cyclized nitrile group in the pre-stabilized precursor. Turning to the carbon fiber manufacturing process, the reduction of energy released during processing of the PAN precursor will allow better control of further oxidative exothermic reactions, thus enhancing the safety of carbon fiber manufacturing.

Oxidation by oxygen

The pre-stabilized precursor is exposed to an oxygen-containing atmosphere to form a stabilized precursor. Thus, the pre-stabilized precursor is converted to a stabilized precursor. This step of embodiments of the processes described herein may also be referred to herein as an "oxidation" or "oxidizing" step. The conditions for forming the stabilized precursor are discussed below.

During the oxidation step, the pendant nitrile groups in the PAN precursor that were not cyclized during the pre-stabilization step may now undergo further cyclization. Thus, the oxidation step increases the amount of nitrile groups cyclized (and thus the amount of hexagonal carbon-nitrogen rings) relative to the pre-stabilized precursor fiber, resulting in a higher proportion of ladder structures in the precursor. By increasing the amount of nitrile groups cyclized, the precursors obtain increased thermal stability and are suitably prepared for use in subsequent carbonization processes described herein, which can be used to form carbon-based materials such as carbon fibers.

Stabilized precursors comprising a high proportion of cyclized nitrile groups can be beneficial in enabling the formation of high quality carbon materials having desirable physical and tensile properties. In some embodiments, the stabilized precursor may comprise at least 50% cyclized nitrile groups, preferably at least 60% cyclized nitrile groups. The stabilized precursor may contain up to about 85% of cyclized nitrile groups. In particular embodiments, the stabilized precursor may comprise from about 65% to 75% cyclized nitrile groups.

By forming a pre-stabilized precursor comprising at least 10% cyclized nitrile groups, a desired amount of cyclized nitrile groups can be obtained in the stabilized precursor in less time and with concomitant lower energy consumption and cost.

The skilled person will understand that during the oxidation step, further chemical reactions such as dehydrogenation and oxidation reactions as well as intermolecular cross-linking reactions may also occur. Dehydrogenation reactions along the polymer backbone can result in the formation of conjugated electron systems and fused ring structures, while oxidation reactions can result in the formation of carbonyl and hydroxyl functional groups.

The oxygen-containing atmosphere to which the pre-stabilized precursor is exposed during the oxidation step comprises a suitable amount of oxygen.

The oxygen-containing atmosphere may comprise only oxygen (i.e. molecular oxygen or O)₂) Or it may contain oxygen in combination with one or more of the gases in the blend. In some embodiments, the oxygen-containing atmosphere has an oxygen concentration of from about 5% to 30% by volume.

In one embodiment, the oxygen-containing atmosphere is air. The skilled person will understand that the oxygen content of air is about 21% by volume.

In one set of embodiments, an oxygen-containing gas stream such as air may be used to establish an oxygen-containing atmosphere.

The exposure of the pre-stabilized precursor to the oxygen-containing atmosphere may be carried out for a desired period of time and at a desired temperature sufficient to form a stabilized precursor. Furthermore, in some embodiments, tension may also be applied to the pre-stabilized precursor during the oxidation step.

Similar to the pre-stabilization step, a number of indicators can be used to guide the selection of process conditions (i.e., temperature, time period, and tension) used during the oxidation step to convert the pre-stabilized precursor to the stabilized precursor. These indices may be considered individually or in combination. The oxidation process conditions can be selected to facilitate formation of a stabilized precursor fiber having desired properties.

In some embodiments, the selection of oxidation process conditions for converting a pre-stabilized precursor to a stabilized precursor may depend on the desired outcome associated with one or more of the following criteria being generated in a fully stabilized precursor: mechanical properties of the precursor (tensile properties including ultimate tensile strength, tensile modulus and elongation at break), precursor fiber diameter, mass density, degree of nitrile group cyclization (% EOR), and appearance. The process conditions employed during oxidation may be adjusted so as to promote the evolution of one or more of the above criteria to achieve the desired result in the stabilized precursor produced at the end of the oxidation step.

In some embodiments, it may be desirable for the process conditions employed during the oxidation step to be selected to produce a stabilized precursor having the desired tensile properties.

For example, in some embodiments, it may be desirable for the process conditions employed during the oxidation step to be selected so as to produce a minimum of ultimate tensile strength and/or tensile modulus in the stabilized precursor produced by the oxidation step, as low tensile strength and tensile modulus may provide an indication of a high degree of precursor stabilization.

Further, in some embodiments, it may be desirable for the process conditions employed during the oxidation step to be selected to produce a maximum elongation to break value in the stabilized precursor produced by the oxidation step.

The oxidation process conditions (i.e., temperature, time period, and tension) used to convert the pre-stabilized precursor to the stabilized precursor can be selected to suitably promote chemical reactions, including nitrile group cyclization and dehydrogenation, during the oxidation step, which facilitates the formation of a stabilized precursor having desirable tensile properties.

As an example, it has been found that at fixed temperature and time conditions during the oxidation step, the properties of ultimate tensile strength and tensile modulus of the PAN precursor may each decrease as the amount of tension applied to the pre-stabilized precursor increases. The decrease in ultimate tensile strength and tensile modulus continues until a minimum value for each property is reached. Thereafter, a further increase in the amount of tension applied to the precursor results in an increase in the ultimate tensile strength and tensile modulus.

Similarly, at fixed temperature and time conditions during the oxidation step, the elongation at break of the stabilized PAN precursor may increase as the amount of tension applied to the pre-stabilized precursor during oxidation increases until a maximum elongation at break value is achieved. Above the maximum, the elongation at break will start to decrease relative to a corresponding increase in applied tension. In some embodiments, it may be desirable that the process conditions employed during the oxidation step be selected so as to produce a maximum elongation to break value in the stabilized precursor formed by the oxidation step.

The precursor fiber diameter may also be reduced as a result of the oxidation step. The reduction in fiber diameter is the result of a combination of weight loss and fiber shrinkage induced by chemical reactions. In some embodiments, the diameter of the fiber may be affected by the tension applied to the precursor during the oxidation step.

As the stabilization and evolution of the ladder structure progresses during the oxidation step, the mass density of the precursor increases during the oxidation and may follow a linear trend. Thus, the mass density of the fully stabilized precursor can be used as an indicator to help guide the selection of process conditions for the oxidation step.

In some embodiments, the process conditions selected for the oxidation step are sufficient to form a catalyst having a molecular weight in the range of from about 1.30g/cm³To 1.40g/cm³A mass density in the range of (a). Stabilized precursors having mass densities within such ranges may be suitable for making high performance carbon fibers.

Another indicator that can be used for the selection of oxidation process conditions is the degree of cyclization of the nitrile group (% EOR) in the stabilized precursor. The degree of reaction (% EOR) provides a measure of the proportion of cyclic structures in the stabilized precursor. This index, combined with knowledge of the% EOR produced during the pre-stabilization step, may allow for determining how much cyclization occurred during the oxidative stabilization process.

In some embodiments, the process conditions selected for the oxidation step are sufficient to form a stabilized precursor having at least 50% cyclized nitrile groups, preferably at least 60% cyclized nitrile groups. The stabilized precursor may have up to about 85% cyclized nitrile groups. In one set of embodiments, the process conditions selected for the oxidation step are sufficient to form a stabilized precursor having from about 65% to 75% cyclized nitrile groups. The degree of cyclization of the nitrile group in the stabilized precursor was determined using FT-IR spectroscopy according to the procedure described herein.

One advantage of the process of the present invention is that stabilized precursors having at least 60%, preferably at least 65%, of cyclized nitrile groups can be formed rapidly in a shorter period of time than alternative stabilization processes

In some embodiments, a low density stabilized precursor may be formed by the stabilization process of the present invention. It has been found that a low density, stabilized precursor can be formed by subjecting a pre-stabilized precursor as described herein to oxidative stabilization conditions as described herein. Such low density stabilized precursors can have at least 60%, at least 65%, or at least 70% cyclized nitrile groups and a nitrile group content of from about 1.30g/cm³To 1.33g/cm³Mass density in the range of (1). It has been found that such low density stabilized precursors are sufficiently thermally stable and can be carbonized and converted to carbon-based materials such as carbon fibers having acceptable properties. It is believed that the stabilization process of the present invention can produce unique low density stabilized precursors because the process utilizes a pre-stabilization step to form a pre-stabilized precursor prior to oxidative stabilization.

An additional indicator that can be used to help guide the selection of oxidation process conditions is the appearance of a fully stabilized precursor. For example, it may be desirable to select process conditions to limit or avoid the formation of a sheath-core cross-sectional morphology in the stabilized precursor, as sheath-core formation is a result of non-uniform stabilization of the precursor from its sheath to its core. However, in some embodiments, the fully stabilized precursor formed according to the process of the present invention may have a sheath-core cross-sectional morphology. Furthermore, fully stabilized PAN precursors prepared according to embodiments described herein are preferably substantially defect-free and have an acceptable appearance. It is believed that defects including melting of the precursor or partial strand breakage may lead to low tensile properties or even failure of carbon materials prepared with the stabilized precursor.

The stabilized precursor formed according to the stabilization process of the present invention is thermally stable and resistant to combustion when exposed to an open flame. In addition, the stabilized precursor can be carbonized for conversion to carbon-based materials such as carbon fibers.

In one set of embodiments, the oxidation step may be performed at room temperature (about 20 ℃), but is preferably performed at an elevated temperature.

For PAN precursors that have undergone pre-stabilization, the oxidation step may be performed at a lower temperature than that typically used to produce the stabilized precursor.

In some embodiments of the precursor stabilization processes described herein, the oxidation step used to form the stabilized precursor can be performed at a temperature at least 20 ℃ lower than the temperature used in conventional stabilization processes or alternative stabilization processes that do not utilize a pre-stabilization step.

The ability to perform the oxidation step at lower temperatures can be advantageous because it can help reduce the risks associated with uncontrolled heat release and thermal runaway that can result from chemical reactions that occur during precursor stabilization. Furthermore, by lowering the temperature at which the oxidation step is carried out, the amount of energy required to stabilize the precursor can also be reduced.

For example, it is believed that the pre-stabilized precursor is sensitive to oxygen and is in an "activated state" whereby it is reactive to oxygen. This can therefore shorten the time period required for precursor stabilization, which will result in significant energy savings and manufacturing cost reductions.

In particular, when pre-stabilized precursors having a high content of cyclized nitrile groups are exposed to an oxygen-containing atmosphere, it has been found that the oxidation reaction leading to complete stabilization of the precursor can be completed in a shorter period of time. Thus, by initially forming a pre-stabilized precursor having at least 10%, at least 15% or at least 20% of cyclized nitrile groups, the rate of oxidative stabilization reactions and further cyclization of nitrile groups in the precursor can be increased when the pre-stabilized precursor is exposed to an oxygen-containing atmosphere, thus enabling a reduction in the time period required to form the stabilized precursor.

In some embodiments, the oxidizing step is performed at an elevated temperature.

In one embodiment, the pre-stabilized precursor is heated in an oxygen-containing atmosphere when the oxidation step is performed. The oxygen-containing atmosphere may contain a suitable amount of oxygen. In a preferred embodiment, the oxygen-containing atmosphere comprises at least 10% oxygen by volume. In one embodiment, the oxygen-containing atmosphere is air.

One skilled in the art will appreciate that the oxidation stabilization reaction that occurs during the oxidation step may consume oxygen atoms. Thus, the oxygen content in the oxygen containing atmosphere may be less than the oxygen content in the gas used to establish the oxygen containing atmosphere.

In a preferred embodiment, the pre-stabilized precursor is heated in air to form the stabilized precursor.

The oxidation step may be carried out at a temperature higher or lower than the temperature of the pre-stabilization step. Alternatively, the oxidation step may be performed at approximately the same temperature as that used for the pre-stabilization step.

In particular embodiments, the pre-stabilized precursor is heated in an oxygen-containing atmosphere at a temperature below the temperature used to form the pre-stabilized precursor. That is, the oxidation step may be performed at a temperature lower than the temperature of the pre-stabilization step.

In one form, the oxidation step is conducted at a temperature above ambient room temperature and below the temperature used to form the pre-stabilized precursor in the pre-stabilization step.

In some embodiments, the pre-stabilized precursor may be heated in an oxygen-containing atmosphere at a temperature at least 20 ℃ lower than the temperature used in the pre-stabilization step.

In a preferred embodiment, the pre-stabilized precursor is heated in an oxygen-containing atmosphere at a temperature in the range of from about 200 ℃ to 300 ℃.

When the oxidation step is carried out at an elevated temperature, the pre-stabilized precursor may be heated at a substantially constant temperature profile or a variable temperature profile.

In one set of embodiments, the pre-stabilized precursor is heated at a constant temperature profile. In such embodiments, the pre-stabilized precursor may be heated at a temperature of about 300 ℃.

In one set of embodiments, the pre-stabilized precursor is heated at a variable temperature profile. For example, the pre-stabilized precursor may be initially heated at a selected temperature, and then the temperature may be increased as the oxidation step proceeds. As an example, the pre-stabilized precursor may be initially heated at a temperature of about 230 ℃, with the temperature increasing to about 285 ℃ during the oxidation step.

Since the oxidation step may be exothermic, it may be desirable to perform the oxidation step at a controlled rate. This can be achieved in a number of ways, for example by passing the pre-stabilised precursor through a series of temperature zones having progressively increasing temperatures within the desired temperature range.

The flow path of the pre-stabilized precursor may be such that the precursor passes through a particular temperature zone in a single pass or in multiple passes.

In some embodiments, heating of the pre-stabilized precursor during the oxidation step may occur by passing the pre-stabilized precursor through a single temperature zone.

In other embodiments, heating of the pre-stabilized precursor during the oxidation step may occur by passing the pre-stabilized precursor through multiple temperature zones. In such embodiments, the pre-stabilized precursor may pass through 2,3, 4, or more temperature zones. Each of the zones may have the same temperature. Alternatively, two or more zones may have different temperatures. For example, at least one temperature zone (e.g., a first temperature zone) may be at a first temperature while at least one temperature zone (e.g., a second temperature zone) is at a second temperature different from the first temperature.

The pre-stabilized precursor may pass through a given temperature zone in a single pass, or it may pass through a given temperature zone in multiple passes. Multiple passes through the temperature zone may be used to increase the time for exposing the pre-stabilized precursor to an oxygen-containing atmosphere.

In some embodiments, each temperature zone may provide an oxidation zone in which a reaction is conducted that promotes stabilization of the pre-stabilized precursor.

In embodiments, the oxygen-containing gas stream may be heated when the oxygen-containing gas stream is used to establish an oxygen-containing atmosphere. The heated oxygen-containing gas stream may be used to bring the pre-stabilized precursor to reaction temperature.

As discussed above, the pre-stabilized precursor may be activated for the oxidation step due to cyclization of a portion of the nitrile group in the PAN precursor during the pre-stabilization step. In particular, it has been found that activating the precursor by a pre-stabilization step may enable a more rapid formation of a stabilized precursor.

In one set of embodiments, the pre-stabilized precursor is exposed to an oxygen-containing atmosphere for a period of time selected from the group consisting of: no more than about 60 minutes, no more than about 45 minutes, no more than about 30 minutes, and no more than about 20 minutes.

The present invention may provide a process for rapidly preparing a stabilized PAN precursor fiber capable of being carbonized to form a carbon fiber, wherein the process (comprising a pre-stabilization step and an oxidation step) is carried out for a period of time selected from the group consisting of: no more than about 60 minutes, no more than about 45 minutes, no more than about 30 minutes, no more than about 25 minutes, or no more than about 20 minutes.

Thus, a stabilized precursor fiber suitable for carbon fiber manufacture can be formed within a time period selected from the group consisting of: no more than about 60 minutes, no more than about 45 minutes, no more than about 30 minutes, no more than about 25 minutes, or no more than about 20 minutes.

The ability to rapidly form a stabilized precursor that can be carbonized can provide significant time, energy, and cost savings in the manufacture of carbon-based materials, such as carbon fibers. For example, a stabilized precursor having a desired amount of cyclized nitrile groups can be formed at least 25%, at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, or at least 80% faster than a comparative stabilization process designed to form a similarly stabilized precursor, but not including the pre-stabilization step described herein.

One advantage is that the oxidation step for precursor stabilization can be performed at a high rate. This may help reduce the impact of the oxidation step on the time and cost of carbon fiber manufacture.

In one set of embodiments, the pre-stabilized precursor is exposed to an oxygen-containing atmosphere under tension. This means that a predetermined amount of tension is applied to the pre-stabilized precursor during the oxidation step. In a particular embodiment, the pre-stabilized precursor is heated under an applied tension in an oxygen-containing atmosphere. The tension applied during the oxidation step may help facilitate the chemical reactions that occur during stabilization, enhance the molecular arrangement of polyacrylonitrile, and allow the formation of more highly ordered structures in the precursor.

In one set of embodiments, a tension in the range of from about 50cN to 50,000cN or from about 50cN to 10,000cN is applied to the pre-stabilized precursor during the oxidation step.

In one set of embodiments, the pre-stabilized precursor is exposed to an oxygen-containing atmosphere at a predetermined temperature for a predetermined period of time.

The predetermined temperature may be a temperature in the range from room temperature (about 20 ℃) up to about 300 ℃, preferably a temperature in the range from about 200 ℃ to 300 ℃.

The predetermined period of time may be selected from the group consisting of: no more than about 60 minutes, no more than about 45 minutes, no more than about 30 minutes, and no more than about 20 minutes.

When the pre-stabilized precursor is exposed to an oxygen-containing atmosphere at a predetermined temperature for a predetermined period of time, a tension may be applied to the pre-stabilized precursor while in the oxygen-containing atmosphere in order to promote the evolution of one or more of the above-described criteria and thus help form a stabilized precursor having desired properties suitable for carbon fiber manufacture.

The oxidation step may be carried out in a suitable oxidation reactor. Suitable oxidation reactors may include an oxidation chamber adapted to stabilize a pre-stabilized precursor in an oxygen-containing atmosphere; an inlet for allowing the pre-stabilized precursor to enter the oxidation chamber; an outlet for allowing the pre-stabilized precursor to exit the oxidation chamber; and a gas delivery system for delivering an oxygen-containing gas to the oxidation chamber. In one set of embodiments, the oxygen-containing gas is air.

Suitable oxidation reactors include conventional oxidation reactors known in the art. In accordance with the stabilization process of one or more embodiments described herein, the operating parameters of the oxidation reactor may be adjusted to oxidize the pre-stabilized precursor and form a stabilized precursor.

An exemplary oxidation reactor may be a furnace or oven adapted to contain an oxygen-containing atmosphere, such as air.

Multiple oxidation reactors may be used to perform the oxidation step.

The oxidation reactor may comprise a single oxidation chamber or a plurality of oxidation chambers. In case the reactor comprises a plurality of oxidation chambers, the pre-stabilized precursor may be transported from one oxidation chamber to the next by suitable transport means.

The multiple oxidation chambers used to perform the oxidation step may be at the same temperature, or two or more chambers may be at different temperatures. Each reaction chamber may provide a temperature zone in which oxidation of the pre-stabilized precursor may be performed.

The oxygen-containing gas stream may be used to establish an oxygen-containing atmosphere in the oxidation chamber. Exothermic energy can be released when the nitrile group in the pre-stabilized precursor undergoes cyclization and through oxidation reactions. Thus, the oxygen-containing gas stream may help dissipate the exothermic energy released during the oxidation step.

The pre-stabilized precursor is delivered through an oxidation reactor comprising one or more oxidation chambers to form a stabilized precursor. The pre-stabilized precursor may be delivered such that it passes through each oxidation chamber in a single pass or in multiple passes at a predetermined rate. The length of the oxidation reactor, the flow path of the precursor through each oxidation chamber, and the rate at which the precursor is delivered through each chamber may each affect the total residence time of the precursor in the oxidation reactor. Further, the residence time may determine the time period during which the oxidation step is performed.

Furthermore, the total residence time of the pre-stabilized precursor in the oxidation reactor may be affected by the temperature within each oxidation chamber, and vice versa. For example, in embodiments where a higher temperature is used for oxidation, it may be desirable to shorten the residence time in the oxidation reactor as compared to embodiments where a lower temperature is used.

In one set of embodiments, the residence time of the pre-stabilized precursor in the oxidation reactor is no more than 60 minutes or no more than about 45 minutes, no more than about 30 minutes, or no more than about 20 minutes.

For a given oxidation reactor, the temperature of each oxidation chamber, as well as the rate at which precursor is delivered through each chamber and the flow path of the precursor, may be adjusted in order to achieve a desired residence time.

In some embodiments, the rate at which the precursor is delivered through the oxidation reactor is selected to match the linear velocity used during the pre-stabilization step. This may allow the pre-stabilized precursor formed in the pre-stabilization step to be fed directly to the downstream oxidation step. This may therefore avoid the need to collect the pre-stabilised precursor. Thus, an oxidation reactor for oxidizing the pre-stabilized precursor to form the stabilized precursor may be located downstream of the pre-stabilization reactor.

In some embodiments, the pre-stabilized precursor may be conveyed through the oxidation reactor at a velocity in a range from about 10 meters per hour to 1,000 meters per hour.

In some particular embodiments, during the oxidation step, the pre-stabilized precursor is heated in an oxygen-containing atmosphere at a temperature in the range from about 200 ℃ to 300 ℃. Heating at a temperature within this range may occur for a period of time selected from the group consisting of: no more than about 60 minutes or no more than about 45 minutes, no more than about 30 minutes, or no more than about 20 minutes.

The oxidation reactor may include one or more heating elements to facilitate heating the pre-stabilized precursor to a desired temperature. In some embodiments, the heating element may heat an oxidation chamber adapted to oxidize the pre-stabilized precursor. Alternatively or additionally, the oxidation reactor may comprise one or more heating elements that heat the oxygen-containing gas stream delivered to the oxidation chamber. The heated gas flow may help control the temperature of the precursor as it passes through the oxidation chamber. The heated gas flow may also help to promote the diffusion of oxygen through the pre-stabilized precursor, help to control excess heat induced by the chemical exothermic reaction through controlled shaking of the precursor, and also help to carry away toxic gases emitted as a result of the chemical reactions taking place in the precursor during the oxidation step.

As discussed above, in some embodiments, tension is also applied to the pre-stabilized precursor during the oxidation step. The desired amount of tension may be applied by tensioning devices located upstream and downstream of each oxidation chamber used for stabilization of the pre-stabilized precursor. The precursor is suspended between tensioners adapted to pass the precursor through each oxidation chamber under a predetermined amount of tension.

In some embodiments, the tensioning device is a material processing device (such as those known in the art) and is a separate component from the oxidation chamber. Examples of material handling devices include drive rollers.

In some embodiments, the oxidation reactor will include one or more of the tensioners. In embodiments where the oxidation reactor comprises two or more oxidation chambers, a tensioning device may be provided upstream and downstream of each oxidation chamber such that the precursor is delivered via the tensioning device as it passes from one oxidation chamber to the next.

The tensioning device may be controlled by a tension controller to enable a predetermined amount of tension to be applied to the pre-stabilized precursor fiber. The amount of applied tension can be monitored by using a tensiometer or an electrical pressure measuring element.

Similar to the pre-stabilization, once the processing parameters of temperature, time and tension are selected for the oxidation of the pre-stabilized precursor, these parameters remain fixed and unchanged while the oxidation step is performed. In addition, the controller may be used to ensure that the process parameters are adequately maintained within acceptable limits of the selected values. This may help ensure that consistent and stable precursor stabilization can be achieved.

In one set of embodiments, a continuous process for preparing a stabilized precursor is provided. In such embodiments, the pre-stabilization step and the oxidation step are performed in a continuous manner. That is, the oxidation step is performed immediately after the pre-stabilization step.

In another aspect, the present invention provides a continuous process for preparing a stabilized precursor fiber for a carbon fiber, the process comprising the steps of:

feeding a precursor comprising polyacrylonitrile to a pre-stabilization reactor comprising a substantially oxygen-free atmosphere, and heating the precursor in the substantially oxygen-free atmosphere while applying a substantially constant amount of tension to the precursor to promote cyclization of nitrile groups in the precursor, the temperature and period of time in which the precursor is heated in the atmosphere and the amount of tension applied to the precursor each being selected to form a pre-stabilized precursor having at least 10% cyclized nitrile groups as determined by fourier transform infrared (FT-IR) spectroscopy; and

the pre-stabilized precursor is fed to an oxidation reactor containing an oxygen-containing atmosphere, and the pre-stabilized precursor is exposed to the oxygen-containing atmosphere to form a stabilized precursor.

In some embodiments, the pre-stabilized precursor has at least 15% or at least 20% cyclized nitrile groups as determined by fourier transform infrared (FT-IR) spectroscopy.

In a specific embodiment, the pre-stabilized precursor has from 20% to 30% cyclized nitrile groups as determined by fourier transform infrared (FT-IR) spectroscopy.

In embodiments of the continuous stabilization process described herein, the oxidation reactor is located downstream of the pre-stabilization reactor.

Depending on the temperature, it may be desirable for some practical considerations to cool the pre-stabilized precursor before it is exposed to the oxygen-containing atmosphere. For example, the pre-stabilized precursor may be cooled to a temperature below the ambient temperature of the oxidation reactor prior to being fed to the oxidation reactor. In the case where the oxidative stabilization is carried out in an oxidation reactor comprising a plurality of temperature zones, the pre-stabilized precursor may be cooled to a temperature below the temperature of the first temperature zone of the oxidation reactor.

Cooling of the pre-stabilized precursor results in the transfer of heat from the pre-stabilized precursor.

Cooling of the pre-stabilized precursor may be particularly desirable to limit the risk of fire that may occur if the pre-stabilized precursor is at a temperature above the temperature of the oxygen-containing atmosphere in the oxidation reactor.

In some embodiments, the pre-stabilized precursor is cooled to a temperature selected from the group consisting of less than 240 ℃, less than 140 ℃, and less than 100 ℃.

For safety reasons, a temperature of less than 240 ℃ of the pre-stabilized precursor may be desirable to at least limit or avoid the risk of fire.

A temperature of less than 140 ℃ may be desirable to ensure that the pre-stabilized precursor is lower in exotherm than the pre-stabilized precursor, as determined by Differential Scanning Calorimetry (DSC). This may help to properly limit the reaction of the pre-stabilized precursor with oxygen in the ambient atmosphere before the pre-stabilized precursor enters the oxidation reactor.

A temperature of less than 100 ℃ for the pre-stabilized precursor may be desirable to enable handling of the pre-stabilized precursor.

While cooled, it is preferred that the pre-stabilized precursor be kept hot enough for efficient reaction in the oxidizing environment within the oxidation reactor.

Cooling of the pre-stabilized precursor may be achieved by passing the pre-stabilized precursor through a cooling zone before the pre-stabilized precursor enters the oxidation reactor.

In one embodiment, the cooling zone may be provided by a cooling chamber positioned between the pre-stabilization reactor and the oxidation reactor.

In an alternative embodiment, the cooling zone may be part of the pre-stabilization reactor and is provided by a cooling zone within the pre-stabilization reactor. In such embodiments, the cooling zone may be designed to cool the pre-stabilized precursor before it exits the pre-stabilization reactor.

The cooling of the pre-stabilized precursor in the cooling zone may be achieved by active means or passive means.

In some embodiments, the active cooling of the pre-stabilized precursor may include passing a substantially oxygen-free gas stream, such as nitrogen gas, over or around the pre-stabilized precursor. In one embodiment, cooling of the pre-stabilized precursor may be achieved by flowing a substantially oxygen-free cooling gas over or around the pre-stabilized precursor. The cooling gas has a temperature lower than the temperature of the pre-stabilized precursor. In some embodiments, the cooling gas may be at a temperature in a range from about 20 ℃ to about 240 ℃. However, it will be appreciated that this may depend on the temperature of the oxidation reactor into which the pre-stabilised precursor will enter, with the temperature of the cooling gas being selected such that it is relatively cooler than the pre-stabilised precursor. In some embodiments, the pre-stabilized precursor may be exposed to a suitable cooling gas for a predetermined period of time at ambient room temperature in order to cool the pre-stabilized precursor prior to introducing the pre-stabilized precursor into the oxidation reactor.

In other embodiments, active cooling of the pre-stabilized precursor may be achieved by flowing a substantially oxygen-free gas at an appropriate temperature over or around the pre-stabilized precursor at a flow rate or volume that promotes the transfer of heat from the pre-stabilized precursor.

In other embodiments, active cooling of the pre-stabilized precursor may be achieved by passing the pre-stabilized precursor through a cooling chamber or section having a cooled inner surface that cools the atmosphere within the cooling chamber or section. Further, the cooled atmosphere is used to cool the pre-stabilized precursor. The coolant may be used to cool the inner surface. In some embodiments, the cooled inner surface may be used in conjunction with a substantially oxygen-free cooling gas to cool the hot pre-stabilized precursor to a desired temperature.

In some embodiments, passive cooling of the pre-stabilized precursor may include passing the pre-stabilized precursor through a cooling zone, which is a void or volume of space that facilitates the transfer of heat from the pre-stabilized precursor.

The continuous precursor stabilization process employs a pre-stabilization step and an oxidation step as described herein above.

When a continuous process for forming the stabilized precursor is performed, the PAN precursor and the pre-stabilized PAN precursor are preferably fed into the pre-stabilization reactor and the oxidation reactor at substantially the same rate or speed. That is, a common rate or speed is preferably used.

The line speed on the production line may be selected such that the PAN precursor and the pre-stabilized precursor are fed at a rate that enables the precursor and the pre-stabilized precursor to have a desired residence time in the pre-stabilization reactor and the oxidation reactor, respectively.

In one set of embodiments, the line speed is such that the PAN precursor has a residence time (i.e., residence time) in the pre-stabilization reactor of no more than about 5 minutes, no more than about 4 minutes, no more than about 3 minutes, or no more than about 2 minutes.

In one set of embodiments, the line speed is such that the pre-stabilized precursor has a residence time (i.e., residence time) in the oxidation reactor of no more than about 60 minutes, no more than about 45 minutes, no more than about 30 minutes, or no more than about 20 minutes.

In one set of embodiments, the conditions are selected such that the stabilization process (including the pre-stabilization step and the oxidation step) is completed within a time period selected from the group consisting of: no more than about 60 minutes, no more than about 45 minutes, no more than about 30 minutes, no more than about 25 minutes, and no more than about 20 minutes. Thus, a fully stabilized precursor is formed within the time periods mentioned above.

The temperature to which the precursor is subjected during the pre-stabilization and oxidation steps, as well as the tension applied to the precursor during its residence time in the pre-stabilization and oxidation reactors, may also facilitate rapid formation of a stabilized precursor suitable for use in the manufacture of carbon materials such as carbon fibers.

The stabilization process of the embodiments of the invention described herein allows for the formation of stabilized precursors suitable for carbon fiber manufacture in a shorter period of time compared to the period of time of conventional PAN precursor stabilization processes. Only short residence times of the precursors in the pre-stabilization reactor and the oxidation reactor may be required.

One advantage of the processes described herein is that the stabilized precursor can be prepared in a shorter period of time than conventional precursor stabilization processes. A faster stabilization time may be achieved by subjecting the PAN precursor to an initial pre-stabilization step for a very short period of time (e.g., a period of time of no more than about 5 minutes, no more than about 4 minutes, no more than about 3 minutes, or no more than about 2 minutes) and then to an oxidation step that completes the stabilization of the precursor and results in the formation of a stabilized precursor.

An additional advantage is that the oxidation step can also be carried out for a shorter period of time and/or at lower temperatures and energies than conventional oxidative stabilization processes.

Thus, the inclusion of a pre-stabilization step can significantly reduce the total precursor stabilization time, and after additional thermal treatment of the stabilized precursor, carbon-based materials such as carbon fibers can be produced with excellent properties. Thus, a rapid oxidative stabilization of the PAN precursor suitable for manufacturing carbon fibers can be achieved.

The stabilization process described herein may be applied to a range of PAN precursors of varying morphology and composition to form stabilized precursors.

The present invention also provides a stabilized precursor prepared by the stabilization process of any of the embodiments described herein. The stabilized precursor may suitably be used to make carbon-based materials such as carbon fibres.

The stabilized precursor prepared by one or more embodiments of the process described herein can have a particle size of at least 1.30g/cm³And 1.40g/cm³The density of (d) in between. Such densities may be suitable for making high performance carbon materials such as high performance carbon fibers.

It has also been found that the stabilized PAN precursors prepared by the stabilization processes described herein exhibit a range of properties that are different from stabilized precursors formed using conventional stabilization processes.

For example, stabilized PAN precursors prepared according to the stabilization process of the present invention have different crystal structures and may exhibit a smaller apparent crystallite size (Lc (002)) relative to a more stabilized PAN precursor formed by a more stabilization process that does not include a pre-stabilization step. In some embodiments, Lc (002) may be at least 20% less than Lc (002) observed for the more stabilized precursor.

In addition, the stabilized PAN precursors prepared by the stabilization process of the present invention have a higher thermal conversion and are formed with a lower exothermic energy generated, as measured by DSC. This highlights the possibility of the stabilization process of the invention in terms of improving the safety of carbon fiber manufacture.

The stabilized precursor prepared by the stabilization process of the present invention was also observed to have a higher dehydrogenation index (CH/CH) as compared to a stabilized precursor formed using a comparative process that did not include a pre-stabilization step₂Ratio). In some embodiments, the dehydrogenation index can be at least 5% or at least 10% higher than the dehydrogenation index of the comparative stabilized precursor. A higher dehydrogenation index is believed to reflect a higher degree of oxidative chemical reaction or higher chemical conversion of PAN precursor during the oxidation step.

If desired, the stabilized precursor produced by the process according to one or more embodiments described herein may be collected and stored in preparation for carbonization or further use. For example, the stabilized precursor may be collected on one or more reels.

As discussed above, the stabilization process of the present invention, including the pre-stabilization step as described herein, enables the formation of stabilized precursors that are sufficiently thermally stable for carbonization in a rapid manner.

The term "rapid" as used with respect to the processes described herein is intended to indicate that the process proceeds more rapidly (i.e., within a shorter period of time) than a reference process designed to achieve the same results, but does not include a pre-stabilization step as part of the process. Thus, the process of the present invention involving pre-stabilization treatment may provide time savings compared to the reference process. As an example, a conventional reference stabilization process can form a stabilized PAN precursor having from 65% to 70% of cyclized nitrile groups within a time period of about 70 minutes. In contrast, some embodiments of the stabilization process of the present invention may enable formation of stabilized precursors having an equivalent amount of cyclized nitrile groups in a period of time as little as about 15 minutes. Thus, the stabilization process of embodiments of the present invention may achieve a time savings of about 55 minutes (or about 78%) relative to the reference process.

Advantageously, the precursor stabilization process of the present invention enables the formation of stabilized precursors in less time and at lower cost.

In some embodiments, the rapid stabilization process of the present invention may be at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, or at least 80% faster than a reference process designed to achieve an equivalent degree of cyclization of nitrile groups in a stabilized precursor, but not including a pre-stabilization step.

The ability to rapidly stabilize PAN precursors also enables energy savings to be realized, as less energy is consumed when performing the stabilization process of the present invention. This in turn may provide flow cost savings for processes such as carbon fiber manufacturing. For example, the stabilization process of the present invention may consume from about 1.1kWh/kg to 2.6kWh/kg on average. This is in comparison to conventional stabilization processes which have an average energy consumption of from about 3.7kWh/kg to 8.9 kWh/kg.

In another aspect, the present invention also provides a low density stabilized precursor comprising polyacrylonitrile having at least 60% cyclized nitrile groups and a nitrile group content of from about 1.30g/cm³To 1.33g/cm³Mass density in the range of (1). In some embodiments, the low density stabilized precursor has at least 65% or at least 70% cyclized nitrile groups. The low density stabilized PAN precursor is thermally stable and can be converted into carbon materials such as carbon fibers with acceptable properties. Conversion to carbon materials such as carbon fibers can be achieved despite the relatively low density of the stabilized precursor.

The low density stabilized PAN precursors as described herein are also lightweight and can be advantageously used in a variety of applications where lightweight stabilized precursors are desired. For example, a low density stabilized precursor may be suitably incorporated into a fabric.

Carbonizing

The stabilized precursor prepared according to the present invention may undergo carbonization to form a carbon-based material or product such as carbon fiber. In certain embodiments, stabilized precursors prepared according to the processes described herein may be suitable for use in the manufacture of high performance carbon fibers.

In some embodiments, the precursor stabilization processes described herein may be incorporated into processes for making carbon fibers to provide improved carbon fiber manufacturing processes.

The rapid precursor stabilization process of the present invention may enable carbon-based materials, such as carbon fibers, to be produced at a faster rate than manufacturing processes utilizing stabilized precursors prepared using conventional stabilization processes.

In one aspect, the present invention provides a process for preparing a carbon-based material, the process comprising the steps of:

providing a stabilized precursor prepared according to the stabilization process of any of the embodiments described herein; and

carbonizing the stabilized precursor to form the carbon-based material.

The carbon-based material may be in a range of forms including fiber forms, yarn forms, web forms, film forms, fabric forms, woven forms, and mat forms. The pad may be a woven or non-woven pad.

In another aspect, the present invention provides a process for preparing a carbon-based material, the process comprising:

a pre-stabilization stage comprising heating a precursor comprising a polyacrylonitrile in a substantially oxygen-free atmosphere while applying a substantially constant amount of tension to the precursor to promote cyclization of nitrile groups in the precursor, the temperature and time period at which the precursor is heated in the substantially oxygen-free atmosphere and the amount of tension applied to the precursor each being selected to form a pre-stabilized precursor having at least 10% cyclized nitrile groups as determined by fourier transform infrared (FT-IR) spectroscopy;

an oxidation stage comprising exposing the pre-stabilized precursor to an oxygen-containing atmosphere to form a stabilized precursor; and

a carbonization stage comprising carbonizing the stabilized precursor to form a carbon-based material.

In a preferred embodiment, the carbon-based material is carbon fiber. To produce carbon fibers, the stabilized precursor may be in the form of fibers, preferably continuous length fibers.

It will be convenient to describe the carbonization step by reference to the formation of carbon fibers from stabilized precursor fibers. However, the skilled person will appreciate that the carbonization step may be carried out on other forms of stabilised precursor, such that a range of different forms (including forms other than fibres) of carbon-based material may be produced.

providing a stabilized precursor fiber prepared according to the stabilization process of any of the embodiments described herein; and

carbonizing the stabilized precursor fiber to form a carbon fiber.

In another aspect, the present invention provides a process for preparing carbon fibers, the process comprising:

a pre-stabilization stage comprising heating a precursor fiber comprising a polyacrylonitrile in a substantially oxygen-free atmosphere while applying a substantially constant amount of tension to the precursor fiber to promote cyclization of nitrile groups in the precursor, the temperature and time period at which the precursor is heated in the substantially oxygen-free atmosphere and the amount of tension applied to the precursor fiber each being selected to form a pre-stabilized precursor fiber having at least 10% cyclized nitrile groups as determined by fourier transform infrared (FT-IR) spectroscopy;

an oxidation stage comprising exposing the pre-stabilized precursor fiber to an oxygen-containing atmosphere to form a stabilized precursor fiber; and

a carbonization stage comprising carbonizing the stabilized precursor fiber to form a carbon fiber.

A range of suitable conditions may be employed in carbonizing the stabilized precursor fiber. The selection of process conditions for the carbonization step may be selected to promote the formation of a carbon material having desired properties and/or structure. In some embodiments, the carbonization process conditions are selected to enable the formation of high performance carbon materials such as high performance carbon fibers. Suitable process conditions may include conventional carbonization conditions known to those skilled in the relevant art.

During the carbonization step, the ladder-like molecular structures formed in the stabilization step become bonded to each other and are modified into graphite-like structures, thereby forming carbon-based structures of the carbon fibers. In addition, during carbonization, volatilization of elements other than carbon also occurs.

In one set of embodiments, the stabilized precursor fiber is heated in a substantially oxygen-free atmosphere during the carbonization step.

In some embodiments, the carbonizing step involves heating the stabilized precursor fiber in a substantially oxygen-free atmosphere at a temperature in the range of from about 350 ℃ to 3000 ℃, preferably from about 450 ℃ to 1800 ℃.

In one set of embodiments, the carbonizing step may include low temperature carbonization and high temperature carbonization.

Low temperature carbonization may include heating the stabilized precursor fiber at a temperature in a range from about 350 ℃ to about 1000 ℃.

High temperature carbonization may include heating the stabilized precursor fiber at a temperature in a range from about 1000 ℃ to 1800 ℃.

In the carbonization step, low-temperature carbonization may be performed before high-temperature carbonization.

During carbonization, the stabilized precursor fibers may be heated at a variable temperature profile to form carbon fibers. For example, the temperature may vary within a defined temperature range for low temperature carbonization and/or high temperature carbonization.

A variable temperature profile for the carbonization step can be achieved by passing the stabilized precursor fiber through a plurality of temperature zones, wherein each temperature zone is at a different temperature. In one set of embodiments, the stabilized precursor fiber may pass through 2,3, 4, or more temperature zones.

The carbonization step is carried out in a substantially oxygen-free atmosphere, which may contain an inert gas. Suitable inert gases may be noble gases such as argon, helium, neon, krypton, xenon, and radium. Further, a suitable inert gas may be nitrogen. The substantially oxygen-free atmosphere may comprise a mixture of inert gases, such as a mixture of nitrogen and argon.

The carbonization step may be carried out for a period of time suitable for producing carbon fibers. In some embodiments, the carbonizing step may be performed for a time period selected from up to 20 minutes, up to 15 minutes, up to 10 minutes, and up to 5 minutes.

In one set of embodiments, the stabilized precursor is heated under tension during the carbonization step. The tension applied during the carbonization step may help control shrinkage of the carbon material and promote the formation of more highly ordered structures in the carbon material.

Tension values as used in conventional carbonization processes for forming carbon materials, such as carbon fibers, may be used for the carbonization step of the processes described herein.

In some embodiments, the selection of the tension to be applied to the stabilized precursor during the carbonization step may depend on the desired result with respect to one or more mechanical properties of the carbon fiber formed from the precursor. The mechanical properties required for carbon fibers may include tensile properties such as ultimate tensile strength, tensile modulus, and elongation at break. The tension applied to the precursor during carbonization may be adjusted so as to promote the evolution of one or more of the above properties to achieve a desired result in the carbon fiber.

The carbonization of the stabilized precursor may be performed in a variety of different carbonization units, including conventional carbonization units known to those skilled in the relevant art. Such units may use operating parameters known in the art for carbonizing the stabilized precursor.

A suitable carbonization unit may comprise at least one carbonization reactor. A carbonization unit comprising a plurality of reactors, such as two or more carbonization reactors, may also be used.

The carbonization reactor is adapted to carbonize the stabilized precursor in a substantially oxygen-free atmosphere. The reactor may include an inlet for allowing the stabilized precursor to enter the carbonization reactor; an outlet for allowing the stabilized precursor to exit the carbonization reactor; and a gas delivery system for delivering a substantially oxygen-free gas to the carbonation reactor to assist in establishing a substantially oxygen-free atmosphere. In one set of embodiments, the substantially oxygen-free gas comprises nitrogen.

The carbonization reactor may also include one or more heating elements. The heating element may heat the substantially oxygen-free gas delivered to the carbonization reactor. The carbonization reactor can be configured to provide a single temperature zone or multiple temperature zones for heating the stabilized precursor passing therethrough.

An exemplary carbonization reactor may be an oven or furnace that is adapted to contain a substantially oxygen-free atmosphere and that can withstand the high temperature conditions typically used for carbon fiber formation. A conventional oven or furnace suitable for carbon fiber manufacture may be used to perform the carbonization step.

When more than one carbonization reactor is used to perform the carbonization step, the individual carbonization reactors may be arranged in series, with the precursor passing through each reactor only in a single pass. For example, the carbonization unit may include a low temperature furnace and a high temperature furnace for performing the carbonization step. The high temperature furnace will typically be located downstream of the low temperature furnace.

The carbonization is performed in a substantially oxygen-free atmosphere, and a substantially oxygen-free gas stream may be used to establish the substantially oxygen-free atmosphere in the carbonization reactor. In a preferred embodiment, the substantially oxygen-free gas is an inert gas. Suitable inert gases may be noble gases such as argon, helium, neon, krypton, xenon, and radium. Further, the inert gas may be nitrogen. The substantially oxygen-free atmosphere may comprise a mixture of inert gases, such as a mixture of nitrogen and argon.

Those skilled in the art will appreciate that the carbonization units will have a defined length determined by the heating length of each reactor, and that the stabilized precursor may be passed through the carbonization units at a predetermined rate. The length of the carbonization unit and the rate at which the precursor is delivered through the carbonization unit can affect the total residence time of the precursor in the unit. Further, the residence time may determine the time period in which the carbonization step is performed.

In one set of embodiments, the stabilized precursor has a residence time in the carbonization unit of no more than about 20 minutes, no more than about 15 minutes, no more than about 10 minutes, or no more than about 5 minutes.

The temperature of one or more carbonization reactors in the carbonization unit and the rate at which the precursor is conveyed through the carbonization unit may be adjusted in order to obtain the carbon material in a desired time.

In some embodiments, the rate at which the precursor is delivered through the carbonization unit is selected to match the line speed used in the pre-stabilization step and the oxidation step described herein. This may facilitate continuous manufacture of carbon materials such as carbon fibers. In particular embodiments, the stabilized precursor may be conveyed through the carbonization unit at a speed in a range from about 10 meters per hour to 1,000 meters per hour.

In order to easily transport the stabilized precursor through the carbonization unit, the precursor will typically have some tension applied to it to ensure that the precursor does not sag or drag as it passes through the carbonization reactor. Furthermore, the tension applied during the carbonization step may help inhibit shrinkage of the carbon material and promote the formation of more highly ordered structures in the carbon material. The tension values used in conventional carbonization processes for forming carbon materials, such as carbon fibers, may be used for the carbonization step of the processes described herein.

The desired amount of tension may be applied by tensioning devices located upstream and downstream of the carbonization unit for carbonizing the precursor. The precursor is suspended between tensioning means adapted to pass the precursor through the carbonization unit under a predetermined amount of tension.

In some embodiments, the tensioning device is a material handling device (such as a material handling device known in the art) and is a separate component from the carbonization unit. Examples of material handling devices include drive rollers.

In some embodiments, the carbonization units will include one or more of the tensioning devices. In embodiments where the carbonisation unit comprises two or more carbonisation reactors, a tensioning device may be provided upstream and downstream of each carbonisation reactor, such that the precursor is conveyed via the tensioning device as it passes from one carbonisation reactor to the next.

In some embodiments, carbon fibers prepared according to the processes described herein may be formed in a time period of no more than about 70 minutes, no more than about 65 minutes, no more than about 60 minutes, no more than about 45 minutes, or no more than about 30 minutes.

The pre-stabilization step, oxidation step, and carbonization step described herein may be performed as part of a continuous process for forming carbon-based materials, particularly carbon fibers.

In another aspect, the present invention provides a continuous process for preparing a carbon-based material, the process comprising the steps of:

feeding a precursor comprising polyacrylonitrile to a pre-stabilization reactor comprising a substantially oxygen-free atmosphere, and heating the precursor in the substantially oxygen-free atmosphere while applying a substantially constant amount of tension to the precursor to promote cyclization of nitrile groups in the precursor, the temperature and period of time at which the precursor is heated in the substantially oxygen-free atmosphere and the tension applied to the precursor each being selected to form a pre-stabilized precursor comprising at least 10% cyclized nitrile groups as determined by fourier transform infrared (FT-IR) spectroscopy;

the stabilized precursor is fed into a carbonization unit, and the stabilized precursor is carbonized in the carbonization unit to form a carbon-based material.

The carbon-based material is suitably carbon fibre. In such embodiments, the polyacrylonitrile-containing precursor is preferably in the form of continuous fibers. Thus, the process for making carbon fibers as described herein may be continuous.

In yet another aspect, the present invention provides a continuous process for preparing carbon fibers, the process comprising the steps of:

feeding a precursor fiber comprising polyacrylonitrile into a pre-stabilization reactor comprising a substantially oxygen-free atmosphere, and heating the precursor in the substantially oxygen-free atmosphere while applying a substantially constant amount of tension to the precursor to promote cyclization of nitrile groups in the precursor, the temperature and period of time at which the precursor is heated in the substantially oxygen-free atmosphere and the tension applied to the precursor each being selected to form a pre-stabilized precursor fiber comprising at least 10% cyclized nitrile groups as determined by Fourier Transform Infrared (FTIR) spectroscopy;

feeding the pre-stabilized precursor fiber into an oxidation reactor comprising an oxygen-containing atmosphere and exposing the pre-stabilized precursor fiber to the oxygen-containing atmosphere to form a stabilized precursor fiber; and

the stabilized precursor fibers are fed into a carbonization unit, and the stabilized precursor fibers are carbonized in the carbonization unit to form carbon fibers.

As discussed above, the pre-stabilization reactor, oxidation reactor, and carbonization unit in one or more aspects of the invention may be part of a system for forming carbon fibers.

Referring to fig. 22, an example of a carbon fiber production system suitable for continuously producing carbon fibers is shown in block diagram form. Carbon fiber production system 90 includes a pre-stabilization reactor 10 for producing a pre-stabilized precursor 81 from a polyacrylonitrile fiber precursor 80.

The fiber source 40 is used to dispense the precursor 80. The plurality of fibers of precursor 80 are dispensed simultaneously as a tow from fiber source 40. After the precursor fibers 80 are dispensed, they are passed through a material handling device 30, such as a tension frame having a plurality of rollers, as is well known in the art. The material treatment device 30 is used in conjunction with the material treatment device 30 downstream of the reactor 10 to apply a predetermined tension to the precursor 80 as the precursor 80 passes through the reactor 10 to form a pre-stabilized precursor 81.

The pre-stabilized precursor 81 is then fed into an oxidation reactor 20, which oxidation reactor 20 may comprise a series of oxidation chambers. The further material processing device 30 is used to suck the pre-stabilized precursor 81 through the oxidation reactor 20. Similar to the pre-stabilization reactor 10, the material processing devices 30 upstream and downstream of the oxidation reactor 20 may be used to apply a predetermined tension to the pre-stabilized precursor 81 as the pre-stabilized precursor 81 passes through the oxidation reactor 20 to form the stabilized precursor 82.

The stabilized precursor 82 is then treated by the carbonization unit 50 to pyrolyze the stabilized precursor 82 and convert it into carbon fibers 83. The carbonization unit 50 comprises one or more carbonization reactors. The carbonization reactor may be an oven or furnace that is adapted to contain a substantially oxygen-free atmosphere and that can withstand the high temperature conditions typically used for carbon fiber formation. Next, surface treatment may be performed at the treatment station 60. The sizing agent may then be applied to the treated carbon fibers 84 at the sizing station 65.

The sized tow of carbon fibers 85 is then wound and/or bundled using a winder 70.

In accordance with one or more aspects of the present invention, operating conditions under which the pre-stabilization, oxidation and carbonization steps of the process may be carried out in a continuous process for forming carbon-based materials, particularly carbon fibers, are described herein above.

When a continuous process for forming carbon fibers is performed, the precursors may be fed to the pre-stabilization reactor, the oxidation reactor, and the carbonization unit at substantially the same rate or speed. Thus, the precursor is continuously transported from one reactor to the next without the need to collect the precursor between reactors.

Line speeds can be as low as 10 meters per hour (m/hr), and can be as high as 1,000 m/hr. For industrial carbon fiber manufacturing processes, line speeds can range from about 100 m/hr to 1,000 m/hr, such as 120 m/hr to 900 m/hr.

Cooling of the pre-stabilized precursor may occur in a cooling zone. The pre-stabilized precursor passes through a cooling zone before it enters the oxidation reactor.

In some embodiments, the active cooling of the pre-stabilized precursor may include passing a substantially oxygen-free gas stream, such as nitrogen gas, over or around the pre-stabilized precursor. In one embodiment, cooling of the pre-stabilized precursor may be achieved by flowing a substantially oxygen-free cooling gas over or around the pre-stabilized precursor. The cooling gas has a temperature lower than the temperature of the pre-stabilized precursor. In some embodiments, the cooling gas may be at a temperature in a range from about 20 ℃ to about 240 ℃. However, it will be appreciated that this may depend on the temperature of the oxidation reactor into which the pre-stabilised precursor is to enter, with the temperature of the cooling gas being selected such that it is relatively cooler than the pre-stabilised precursor. In some embodiments, the pre-stabilized precursor may be exposed to a suitable cooling gas for a predetermined period of time at ambient room temperature in order to cool the pre-stabilized precursor prior to introduction into the oxidation reactor.

The ability to rapidly form a stabilized precursor, according to one or more embodiments described herein, may provide downstream advantages for carbon fiber manufacture, particularly with respect to the time required to form the carbon fiber. Thus, the rate of carbon fiber production on a production line may be increased due to the rapid stabilization process of the present invention, resulting in the ability to produce carbon fibers at a faster rate and/or at a higher volume than conventional carbon fiber manufacturing processes known in the art. Furthermore, the processes described herein may also enable the production of high volume carbon fibers more rapidly on an industrial scale. Thus, the manufacturing costs associated with the manufacture of carbon fibers may be reduced.

Accordingly, the present invention may also provide the use of a stabilized precursor prepared by the stabilization process of any of the embodiments as described herein in the manufacture of carbon-based materials, such as carbon fibers.

As discussed above, carbon fibers prepared according to the processes described herein may be formed in a time period of no more than about 70 minutes, no more than about 65 minutes, no more than about 60 minutes, no more than about 45 minutes, or no more than about 30 minutes.

Although the process herein has been described with reference to the production of carbon fibres, the skilled person will appreciate that the described process may be used to prepare carbon-based materials in non-fibrous form. That is, when the precursor is in a non-fibrous form (e.g., a yarn form, a web form, a film form, a fabric form, a woven form, or a mat form), the carbon-based material formed after carbonization of the stabilized precursor can be in these other forms.

The present invention also provides a carbon fiber produced by the process of any of the embodiments described herein.

Advantageously, carbon fibers produced according to the processes of embodiments of the invention described herein may exhibit tensile properties at least equivalent to those of carbon fibers produced by conventional carbon fiber manufacturing processes employed in the industry.

Furthermore, carbon fibers made from the stabilized precursor prepared according to the stabilization process of the present invention may exhibit a different crystal structure than carbon fibers known in the art. For example, carbon fibers prepared with stabilized precursors made according to the invention described herein can exhibit a larger apparent crystallite size (Lc (002)) relative to carbon fibers formed with more conventional stabilized precursors. In some embodiments, the Lc (002) of the carbon fiber may be at least 20% greater than the Lc (002) observed for carbon fibers obtained from a more stabilized precursor.

The invention will now be described with reference to the following examples. However, it should be understood that these examples are provided by way of illustration of the present invention, and they in no way limit the scope of the present invention.

Examples

Characterization method

Mechanical testing

The mechanical properties of the single fiber samples were tested on a Textechno Favimat + monofilament tensile tester equipped with a "robot 2" sample loader. The instrument automatically recorded the linear density and force spread data for individual fibers loaded into a cassette (25 samples) with a pretension weight (-80-150 mg) attached to the bottom of each fiber. The instrument used a 210cN load cell and had a 4x4mm configuration²The surface area of (a). The clamping force was set to 45N.

Differential scanning calorimetry

The heat induced transitions were measured using a differential scanning calorimeter (DSC, TA, Q200 series) from a TA instrument. Three milligrams of the sample was heated from 100 ℃ to 400 ℃ under a nitrogen and air atmosphere at a heating rate of 20 ℃/min. Using the method developed by Tsai et al (j. -s.tsai, h. -n.hsu, j. mater.sci.lett.,11(1992)1403-1405), the rate of stabilization of polyacrylonitrile fibers was calculated using the following equation:

wherein H₀Is the area under the exothermic peak of the precursor, and H₁Is the area under the exothermic peak of the sample undergoing stabilization. Exothermic heat generationAll areas under the curve were calculated using sigmoid integration from baseline to baseline.

Fourier transform infrared spectroscopy

The chemical changes induced during stabilization of the samples were analyzed using a Bruker Lumos FT-IR microscope equipped with germanium crystals. Each sample was measured using attenuated total reflectance mode (ATR). For each measurement, a uniform pressure was applied between the crystal and the sample. Average of 128 scans and 4cm per measurement^-1Resolution of from 600cm^-1To 4000cm^-1The process is carried out.

The amount of cyclized nitrile group in the stabilized sample was calculated according to the method developed by Collins et al, Carbon,26(1988) 671-679. The amount of cyclized nitrile group is expressed as the degree of reaction (% EOR), which is determined according to the following formula:

wherein Abs (1590) and Abs (2242) are the absorbance of the peaks recorded at the corresponding wavenumbers.

CH/CH known as Dehydrogenation Index (DI)₂The ratio of functional groups was calculated using the method of Nunna, Srinivas et al Polymer definition and Stablity, 125 (2016: 105-₂Measured absorbance of the functional group.

Density of

The mass density of the polyacrylonitrile precursor, stabilized polyacrylonitrile precursor and carbon fiber was measured at 23 ℃ using a mass density according to astm d 1505-10: the Density Gradient column Method of the Standard Test Method for the Density of Plastics by the Density-Gradient Technique is used for measurement by the Density Gradient Technique. Two columns were used. The first column was packed with a mixture of potassium iodide and distilled water solution to characterize the precursor and stabilized fiber with a gradient from 1.17g/cm³To 1.45g/cm³. Another column was used to characterize the density of the carbon fibers produced and was packed with 3-ethyl phosphate and 1, 3-dibromopropane, with a gradient of from 1.60g/cm³To 1.90g/cm³。

X-ray diffraction (XRD)

Wide-angle X-ray diffraction experiments were performed according to the literature procedures (F. Liu et al, effective of microstructure on the mechanical properties of PAN-based carbon fibers during high-temperature mapping, J Mater Sci 43(12 (2008)4316-4322), using a radiation source equipped with Cu-K α

XRD was performed on the sample by X-pert pro PANALYtic XRD of (X-Pert pro). The X-ray tube was set to 40kV and 30 mA. The diffraction peaks of the samples were obtained by performing absolute measurements, in which the point focus was varied between 5 ° and 60 °. Prior to measurement, the fiber samples were arranged on a low noise silicon background. The apparent crystallite size and d-spacing of the samples were obtained using

equations

4 and 5.

Bragg equation n λ 2dsin θ (5)

Where "B" is the full width at half maximum (FWHM) intensity of the diffraction peak corresponding to the different facets studied, and "k" is a constant equal to 0.89. The diffraction angles 2 θ of the PAN precursor and the oxidized fiber were 2 θ ═ 17 ° and 2 θ ═ 25.5 ° for crystal plane (100) and crystal plane (002), respectively. As for the carbon fiber, only the diffraction angle 2 θ corresponding to the crystal plane (002) is considered to be 25.5 °. In equation (5), "d" corresponds to the spacing between crystal planes. Curve fitting was applied in order to calculate the FWHM and center of the diffraction peak. The apparent crystallite size and d-spacing were calculated by taking into account the standard error of the peak center and FWHM values.

Thermogravimetric analysis (TGA)

Weight loss of the samples was determined via thermogravimetric analysis using a TA Instruments Q50 thermogravimetric analyzer (USA). Three milligrams of fiber were tested at a heating rate of 20 ℃/min under a nitrogen atmosphere from 100 ℃ to 600 ℃. The experiments were performed in triplicate and the results are shown as average values.

Experiment of

Comparative example 1(CE-1)Production of stabilized fibers and carbon fibers without pre-stabilization (base line)

The precursor used in this study was commercial Polyacrylonitrile (PAN), which comprised 24000 filaments (1.3 dtex) containing acidic comonomer coated with sizing agent. Carbon fibers were prepared using the Carbon Nexus production line of the university of deacon, australia.

The PAN precursor was stabilized in an air atmosphere by passing through a series of ovens operated with 4 different temperature zones. The total residence time for oxidation was 68 minutes. The material handling actuator is used to control the tension applied to the precursor. The first driver (driver 1) is located before the temperature zone 1. The second driver (driver 2) is located between temperature zone 1 and temperature zone 2. The third driver (driver 3) is located after the temperature zone 2. The fourth driver (driver 4) is located after zone 4.

The stabilized precursor is then carbonized by heating in nitrogen in a low temperature furnace and a high temperature furnace. The cumulative residence time for carbonization was 3.7 minutes. The process parameters are summarized in table 1.

Table 1. process parameters for baseline.

Mechanical testing was performed using Favimat and is shown in table 2.

Table 2 mechanical properties and density of the baseline carbon fibers.

With respect to the mechanical test results, the ultimate tensile strength and modulus gradually decreased along the stabilization process. The opposite behavior is shown, with the elongation of the polymer gradually increasing until zone 3, and then eventually decreasing in zone 4. Density was measured using a density column. The precursor did show 1.182g/cm³Is characteristic of the acrylic precursor. The density recorded for the stabilized fibres was 1.367g/cm³. It has been reported in the literature that in order to produce carbon fibers with optimal mechanical properties, the density of the stabilized fibers should lie at 1.34g/cm³And 1.39g/cm³(Takuku et al, J.Appl.Polym.Sci.,30, (1985), 1565-.

FT-IR technology is used to monitor the chemical changes that occur during stabilization. The FT-IR spectrum of a commercial PAN fiber is characterized by the following characteristic peaks: 2242cm^-1(unreacted nitrile group), 1730cm^-1(C ═ O from acidic comonomer), 1450cm^-1(CH from the Polymer backbone)₂)，1260cm^-1、1090cm^-1And 1022cm^-1(multiple CH groups, CO groups, OH groups). When stabilization occurs, cyclization chemical reactions, dehydrogenation chemical reactions, and oxidation chemical reactions occur. In stabilized fibres at 1590cm^-1And 1365cm^-1The peaks appearing there appear to be related to the cyclization reaction product and the dehydrogenation reaction product. 1730cm^-1And 2242cm^-1Decrease from 1000cm^-1To 1700cm^-1The broad band of (A) forms a stabilizing nomenclature (Quyang et al, Polymer depletion and Stability,93(2008), 1415-. The degree of cyclization reaction (EOR) was calculated for the stabilized fibers and showed a value of 68%.

The stabilized fibers were also subjected to DSC analysis and did show a CI index of 70%.

Example 1Pre-stabilization of PAN precursor fibers in nitrogen using isothermal temperature profile (PSN-1)

The precursor used in this study was commercial Polyacrylonitrile (PAN), which comprised 24000 filaments (1.3 dtex) containing acidic comonomer coated with sizing agent.

The PAN precursor was heated under nitrogen in a reactor, which was a Low Temperature (LT) furnace operating with 4 temperature zones, as illustrated in fig. 1. The temperature of each heating zone was set at 300 ℃ and operated at as high a temperature as possible without causing degradation of the precursor. The line speed was set to provide a residence time in the furnace of 1 minute 10 seconds in the heating and nitrogen atmosphere. The tension applied to the precursor fiber was set to a constant value of 3000cN and adjusted using material handling drives located upstream and downstream of the furnace. The furnace was slightly pressurized with nitrogen to inhibit the ingress of oxygen. After heat treatment in nitrogen, the fibers turned a dark orange/copper color without any signs of degradation or filament fusion.

Favimat was used to record the mechanical properties of the pre-stabilized fiber (PSN-1) and PAN precursors. The results are shown in table 3.

Table 3 mechanical properties and density of PSN-1.

The ultimate tensile strength and tensile modulus are significantly affected by a short nitrogen heat treatment. Indeed, the ultimate tensile strength properties and modulus properties of PSN-1 do show values similar to those encountered in zone 3 of the baseline sample (example CE-1, Table 2). The mass density of the recorded PSN-1 is equal to 1.223g/cm³Similar to the baseline PAN precursor after stabilization in the first zone.

In addition to density measurements, FT-IR spectroscopy was also performed. Fig. 2 shows the FT-IR spectra of PSN-1 samples and the original PAN precursor. After pre-stabilization in nitrogen, the chemical structure of the precursor evolved significantly. Record 1730cm^-1Indicating that the chemical conversion of the acidic comonomer triggered the formation of a cyclized structure consisting of a structure located at 1590cm^-1The appearance of the peak at (a) is highlighted. As typically seen under an air atmosphere at 1590cm^-1(_C＝C,C＝N) Splitting of the singlet at 1616cm^-1(ν_C＝N) Additional peaks were also observed. PAN is not fully conjugated under nitrogen, leading to imine-enamine isomerization by forming dihydropyridine structures. 1400cm in PSN-1 samples as the cyclization reaction progressed^-1Dehydrogenation of the polymer chain was also seen in the case of band formation. The degree of cyclization reaction (EOR) was calculated and a value of 24% was demonstrated.

In addition, the fiber from comparative example (CE-1) having the same coloring as that of the PSN-1 fiber was extracted and tested. To achieve the same coloration in the comparative stabilization process, 5 minutes 30 seconds are required, whereas in the pre-stabilization step the precursor fiber only needs to be treated for 1 minute 10 seconds to achieve a dark orange/copper color. FT-IR analysis was performed on the extracted samples. The% EOR in the fiber treated according to the CE-1 process was calculated and was equal to 2%. Although these fibers have identical colors, it appears that the chemical structure of the PSN-1 fibers is significantly different (2% EOR of baseline versus 24% EOR of PSN-1).

Example 2Pre-stabilization of PAN precursor fibers in nitrogen using a stepwise increasing temperature profile (PSN-2)

The precursor used in this study was commercial Polyacrylonitrile (PAN), which comprised 24000 filaments (1.3 dtex) containing an acidic comonomer coated with a sizing agent.

The PAN precursor was heated under nitrogen using a reactor with a set of heating zones configured to provide 4 temperature zones, as previously described in example 1 and shown in fig. 1. Similar to the process parameters for PSN-1, the line speed was set to provide a residence time of 1 minute 10 seconds in the heating zone. The temperature for

zones

1 and 2 was set to 285 deg.C and the temperature for

zones

3 and 4 was set to 295 deg.C. The tension applied to the fiber was set to a constant amount of 2300 cN. The furnace was slightly pressurized with nitrogen to inhibit the ingress of oxygen.

The mechanical properties of PSN-2 were recorded (Table 4) and indeed showed lower values of ultimate tensile strength and modulus than PSN-1. The measured elongation is higher than PSN-1.

Table 4 mechanical properties and recorded density of PSN-2.

An FT-IR spectrum of PSN-2 was taken and is shown on FIG. 3. PSN-2 has a chemical structure similar to that of PSN-1. PSN-2 pre-stabilized precursor showed a calculated% EOR of 24%.

Example 3Tension vs. PAN precursorEffect of Pre-stabilization

In this study, the effect of strain on the formation of a cyclized structure was studied.

A commercial PAN precursor (24K) comprising an acidic comonomer was used in the following experiments. A reactor configured to provide 4 temperature controlled zones as previously described in example 1 and shown in figure 1 was used. The reactor was purged with high purity nitrogen and over pressurized to avoid any oxygen contact with the fibers.

The linear velocity was set to match the residence time of 1 minute 10 seconds in the heated zone of the reactor. The temperature for

zones

1 and 2 was set to 290 deg.C and the temperature for

zones

3 and 4 was set to 295 deg.C. Three different tensions were selected and applied to the precursor fibers: 2500cN (low), 2700cN (medium), and 3000cN (high), as shown in table 5.

Table 5. processing conditions for the preliminary study of each sample.

It was visually observed that the pre-stabilized precursor fibers adopted different coloration depending on the tension applied to the PAN fibers during pre-stabilization. It was observed that the precursor fiber was colored darker with the highest tension, highlighting the higher degree of chemical reaction.

FT-IR analysis of the raw PAN precursor and the pre-stabilized precursor showed that significant chemical changes were observed after pretreatment in an inert atmosphere. Nitrile Peak (2242 cm)^-1) Reduction of 1610cm^-1(ν_C＝N) The increase in area consistently highlights that cyclization had occurred under nitrogen treatment. At 1730cm^-1The intensity of the peak at (C ═ O functionality) decreases, demonstrating that the cyclization chemistry is initiated by the acidic comonomer. Note that the chemical change becomes more pronounced at higher applied tensions. E.g. with 1730cm^-1Absorption band (v)_C＝O) The decrease in tension shows that as the tension increases, the content of acidic comonomer reacted is higher, which promotes the chemical reaction. For working at different tensionsFor each group of fibers, the degree of cyclization reaction (% EOR) was calculated. The results are shown in table 6. The content of cyclic structures produced after pretreatment was significant in view of the very short residence times used for these experiments.

CH functionality (1360 cm) was also noted on fibers processed at higher tensions^-1) Increase of directly related CH₂Functional group (1450 cm)^-1) Is reduced. CH/CH₂The ratio of functional groups was calculated according to equation (3) and shown in table 6.

TABLE 6 degree of cyclization and CH/CH of Nitrogen pretreated fibers₂A ratio.

Sample name	Tension (horizontal)	EoR(％)	CH/CH₂Ratio of
				PSN-3	Is low in	10.7±0.5	0.46±0.03
PSN-4	Medium and high grade	16.3±0.7	0.58±0.02
				PSN-5	Height of	19.3±0.4	0.65±0.03

Mechanical testing was performed using a Favimat robot, and the results are shown in table 7.

TABLE 7 mechanical properties and densities of PSN-3, PSN-4 and PSN-5.

As the tension increases, the ultimate tensile strength and modulus of the precursor drops significantly from 2300cN to 3000 cN. It was also observed that higher tensions increase the mass density of the precursor.

Differential Scanning Calorimetry (DSC) analysis of the pre-treated PAN fibres was performed under an air atmosphere in order to investigate their thermal behaviour (figure 4). The tension at which the fiber is processed significantly affects the recorded enthalpy, with the enthalpy being lowest at the highest tension used for fiber pretreatment. Interestingly, the thermal behavior of the pre-stabilized samples was different from that of PAN precursors. Pre-stabilization of the precursor in nitrogen gas drastically changes the kinetics of the chemical stabilization reaction. The slope of the exotherm is significantly reduced, which translates into a carbon fiber manufacturing process operating at safer operating conditions.

Example 4Rapid stabilization of the pre-treated PAN precursor within 60 minutes and formation of carbon fibers

The pre-stabilized precursor PSN-1 from example 1 was stabilized using a set of oxidation ovens having 4 temperature zones as described for comparative example 1 (CE-1). The oven arrangement and oxidation process are illustrated in fig. 5. The path of the fiber through the oxidation oven was the same as in example CE-1. Multiple passes through each of the ovens were performed, resulting in PSN-1 being heated in air for 60 minutes to oxidize the pre-stabilized precursor fibers. The stabilized precursor is then carbonized in nitrogen in a low temperature furnace and a high temperature furnace to form carbon fibers. The cumulative residence time for carbonization was 3.1 minutes. The process parameters for oxidation and carbonization are summarized in table 8.

Table 8 process parameters used in example 4.

The oxidation temperature profile used for this experiment was about 20 ℃ lower on each zone than the temperature used to stabilize the precursor in the baseline comparative example (CE-1). This decrease in temperature was observed, although the oxidation residence time to stabilize the fibers was reduced (68 minutes for example CE-1 versus 60 minutes for example 4). The pre-stabilized precursor fibers exhibit a very strong reactivity when exposed to heat and an oxygen-containing atmosphere (oxidation). The pre-stabilized precursor fiber does become completely black after being heated in zone 1. This type of black coloration is typically seen on zone 3 of the baseline process, which is not pre-stabilized.

The evolution of the mechanical properties of PAN fibers along stabilization and carbonization was measured using Favimat. The results are shown in table 9.

Table 9 mechanical properties and recorded density of example 4.

The tensile strength recorded after stabilization of zone 1 of the pre-stabilized PAN fibers is nearly equal to the value of the sample from zone 4 of the baseline test (CE-1). The ultimate tensile strength of the fiber does not evolve sharply over

zones

2,3 and 4. The tensile modulus of carbon fibers generally decreases along the stabilization process (example CE-1). After zone 1, the tensile modulus increased significantly. This type of behavior is typically seen in the carbonization stage of conventional carbon fiber manufacturing processes and can be explained by the different chemical structures employed after nitrogen pre-stabilization. With respect to the evolution of elongation along the stabilization, the baseline showed an increase up to zone 3, and a slight decrease. In this experiment, similar behavior was adopted for elongation, but the drop was recorded after zone 2. The mass density of the stabilized fiber recorded (zone 4) is equal to 1.350g/cm³。

The mechanical properties of the carbon fiber were 3.73GPa and 259GPa for ultimate tensile strength and modulus, respectively.

The heat flow and hence enthalpy under the exotherm of the stabilized fiber (zone 4) was recorded using DSC under a nitrogen atmosphere. Although the stabilized fibers have been processed at a lower temperature for a shorter time, the sample showed a conversion index of 78% which is higher than the index recorded for baseline (70%).

FT-IR spectra of the stabilized sample from baseline (CE-1) and the stabilized PAN precursor (PSNOPF) prepared in example 4 are shown in fig. 6. Along the thermostabilization (baseline), the structure of polyacrylonitrile evolves towards a ladder structure. Nitrile group (2242 cm)^-1) Is converted to a C ═ N group by cyclization and dehydrogenation, yielding a C ═ C group (1590 cm)^-1). When stabilization occurs, the CH initially contained in the backbone of the polymer chain₂Radical (1450 cm)^-1) Essentially converted to CH groups (1370 cm) by cross-linking and dehydrogenation of the polymer chains^-1). After stabilization, various vibration modes of C ═ C group, C ═ O group, C ═ N group, C — C group, C — CN group contained in the stabilized product were seen from 1700cm^-1To 1000cm^-1Is used. Similar changes were seen for the cyclization and crosslinking reactions with respect to the IR spectrum of the stabilized sample. However, significant structural changes were seen substantially at the lowest part of the FT-IR spectrum. At 800cm^-1、1022cm^-1、1260cm^-1Additional or more intense peaks are seen. The formation of these new peaks can be related to the higher amount of aromatic type structures induced by nitrogen pretreatment. The% EOR recorded for the stabilized fiber from example 4 was 69%.

Example 5Rapid stabilization of the pre-treated PAN precursor within 30 minutes and formation of carbon fibers

The pre-stabilized precursor PSN-1 from example 1 was oxidized in a set of ovens operated as illustrated in fig. 7. The linear velocity used in this example was the same as that used in example 4. However, only two heating zones were used in this example, which reduced the oxidation residence time to 30 minutes. The pre-stabilized precursor PSN-1 was passed through each oven in multiple passes. The stabilized precursor is then carbonized in a low temperature furnace and a high temperature furnace to form carbon fibers. The cumulative carbonation residence time was 3.1 minutes. The process parameters for oxidation and carbonization are summarized in table 10.

TABLE 10 Process parameters used in example 5.

Mechanical testing was performed and the results are shown in table 11.

Table 11 mechanical properties and recorded density of example 5.

The ultimate tensile strength and modulus recorded for zone 1 stabilization were higher compared to example 4. The decrease in tensile strength and modulus after zone 2 is similar to the conventional behavior observed with the baseline sample. The density value of the stabilized fiber after oxidation in the two temperature zones was also recorded as 1.343g/cm³. The carbon fibers produced in this industrial test had an ultimate tensile strength of 3.70GPa and a tensile modulus of 244 GPa.

The heat flow and hence the enthalpy under the exotherm of the stabilized fiber (zone 2) was recorded using DSC under a nitrogen atmosphere. The sample shows a conversion index of 78%. The conversion index value registered was higher than the CE-1 baseline sample (70%).

The FT-IR spectra of the stabilized fibers produced after oxidation in

zones

1 and 2 are shown in figure 8. The% EOR recorded for the stabilized fibers was 74%, which is slightly higher than the CE-1 baseline example.

Example 6Rapid stabilization of the pre-treated PAN precursor within 15 minutes and formation of carbon fibers

The pre-stabilized precursor PSN-1 from example 1 was oxidized in a single temperature zone as shown in fig. 9. Similar to the experiment of example 5, the line speed remained the same as that used in example 4. For this example, using only one oxidation oven providing a single oxidation zone allows the oxidation residence time to be reduced to 15 minutes. The pre-stabilized precursor PSN-1 was passed through a single oxidation oven in multiple passes. The stabilized precursor is then carbonized in a low temperature furnace and a high temperature furnace to form carbon fibers. The cumulative carbonation residence time was the same as in examples 4 and 5 (3.1 minutes). The process parameters for oxidation and carbonization are summarized in table 12.

TABLE 12 Process parameters used in example 6.

The mechanical properties of the material along the process were measured using Favimat (table 13).

Table 13 mechanical properties and recorded density of example 6.

High performance carbon fibers having an ultimate tensile strength of 3.56GPa and a modulus of 234GPa were produced in this test. The stabilized fiber from zone 1 in this test had higher mechanical properties than the stabilized fiber from baseline (CE-1). The density value of the stabilized fibre produced by oxidation in a single temperature zone is also recorded and is equal to 1.304g/cm³. Despite the low density of the stabilized fibers, the material is thermally stable enough to be carbonized. This high thermal stability is associated with the enhanced chemical composition of the stabilized fibers formed using this process.

The FT-IR spectrum of the stabilized sample produced after oxidation in a single temperature zone is shown in fig. 10. The spectra were similar to those obtained for example 4 and example 5. 800cm^-1The absorption of (a) is significant, highlighting the formation of aromatic structures at higher processing temperatures. The% EOR of the stabilized sample was 64%. And also proceedDSC experiments, and a CI index of 65% was recorded.

Example 7Rapid stabilization of the pre-treated PAN precursor within 20 minutes and formation of carbon fibers

The pre-stabilized precursor PSN-1 from example 1 was oxidized by passing the pre-stabilized precursor through four different ovens set at different temperatures at a time, as shown in fig. 11. A different tension setting is used for each pass. The pre-stabilized precursor PSN-1 was passed through each oven in a single pass. The residence time of the pre-stabilized fibers was 5 minutes per furnace, so the total oxidation time was 20 minutes. The linear velocity used was the same as that of example 4. The stabilized precursor is then carbonized in a low temperature furnace and a high temperature furnace to form carbon fibers. The cumulative carbonation residence time was the same as that previously used in examples 4, 5 and 6 (3.1 minutes). The process parameters for oxidation and carbonization are summarized in table 14.

TABLE 14 Process parameters used in example 7.

Mechanical testing was performed using Favimat (table 15).

Table 15 tensile properties and recorded density for example 7.

With respect to the results, it was observed that the ultimate tensile strength and modulus of the fibers remained similar along the stabilization process. Similar to example 5, the elongation increased in the first two zones and finally decreased in the last two zones. Note that the density of the carbon fibers produced in this example was higher than the baseline CE-1 example, which may be due to the different structural configuration of the carbon fibers.

FT-IR analysis was performed on the samples after oxidation in each of the different temperature zones (fig. 12). Interestingly, the fiber adopted a similar chemical structure to the stabilized fiber that had been after zone 1. This is evidenced by the% EOR values recorded for the 1-zone elevation. After the pre-stabilization treatment in nitrogen, the fibers are believed to be in an activated state, allowing the oxidative stabilization chemistry to occur more rapidly and to a higher degree.

DSC experiments were also performed on each set of samples from the stabilization process. The CI index was calculated and did show a value of 75% for the stabilized samples.

The results of FT-IR and DSC analyses are shown in Table 16.

Table 16. degree of reaction and conversion index of example 7 during stabilization (oxidation).

Example 8Rapid stabilization and carbonization of different PAN precursor fibers

Different commercial polyacrylonitrile precursors were used in this experiment:

● precursor A: large commercial polyacrylonitrile tow, comprising 50.000 filaments (50K) with an elliptical cross-sectional shape, was covered with a sizing agent. The chemical composition of the precursor includes copolymerized polyacrylonitrile (unknown composition) with an acidic comonomer.

● precursor B: a commercial polyacrylonitrile tow comprising 24.000 filaments (24K) with a round cross-sectional shape, covered with a silicon-based sizing agent. The fiber is made from copolymerized polyacrylonitrile having the following proportions: 93% acrylonitrile, 1% itaconic acid and 6% methyl methacrylate.

Rapid pre-stabilization of PAN precursor fibers under nitrogen atmosphere

Each of precursor a and precursor B was pre-stabilized in a nitrogen atmosphere using a furnace containing 4 heating zones (fig. 1).

In addition to the applied tension, different precursor types were pretreated under the same processing conditions. The tension tested was selected to be appropriate for the tow size. For these tests, the line speed was set to provide a residence time in a nitrogen atmosphere of 1 minute 10 seconds. The temperature profile used for zone 1 and zone 2 was 285 ℃; and the temperature profile used for zone 3 and zone 4 was 295 c, respectively. To suppress any oxygen present, the furnace was slightly pressurized with nitrogen.

To investigate the effect of tension as a process parameter, different precursor fibers were stabilized at different constant applied tension values, as shown in table 17. Samples were characterized using FT-IR, density columns and Favimat.

Table 17 list of samples produced during the pre-stabilization test under nitrogen.

^*Selected pre-stabilized PAN fibers are produced in larger quantities for further stabilization and carbonization in oxygen.

Exposure to oxygen to form stabilized PAN precursor

For each different type of PAN precursor, a larger amount of pre-stabilized fiber was produced for stabilization. The fibers "A-3200" and "C-1600" in Table 17 are candidates selected for further oxidation in air. The setup for the oxidation step was similar for each precursor tested and is illustrated in fig. 11.

Carbonization and formation of carbon fibers

The fully stabilized PAN fibers are carbonized under tension in furnace low and high temperature furnaces under an inert atmosphere. Two furnaces purged with nitrogen were used. The cumulative heating residence time for carbonization was 3.1 minutes. The low temperature furnace has 3 zones set to 450 deg.c, 650 deg.c and 850 deg.c, respectively. The high temperature furnace had 2 zones set at 1100 ℃ and 1500 ℃ respectively. All precursors were processed using the same processing conditions.

Results and discussion

Precursor A

Pre-stabilization in a nitrogen atmosphere

To determine the desired tension to be applied during pre-stabilization, samples of the precursor were initially heated under nitrogen at different tensions from low to high (table 17).

Samples treated at different applied tensions were analyzed by FT-IR and the degree of reaction (% EOR) was measured. The extent of the nitrile cyclization reaction and how cyclization varies with applied tension is shown on figure 13. The curve was observed to adopt a "bell" shape, indicating that there was a maximum intensity (43%) associated with the formation of the ring structure. From this study it appears that there is an amount of applied tension that promotes the optimal amount of cyclized nitrile groups in the PAN precursor. For precursor a, the optimum tension for the precursor is 3000 cN.

The mass density of various pre-stabilized samples was also tested and shown on figure 14 and table 18. The same type of distribution similar to FT-IR was observed. The sample at the maximum of the curve associated with the highest content of cyclic structures (applied tension 3000cN) had the highest mass density recorded.

The mechanical properties of the pre-stabilized fibers were determined using a monofilament tester FAVIMAT. Fig. 15 shows the evolution of the ultimate tensile strength and tensile modulus of pre-stabilized fibers processed at different applied tensions. The results are also presented in table 18. These curves exhibit opposite trends compared to the FT-IR and mass density results. The more stabilization proceeds, the weaker the system becomes. The minimum value of the mechanical properties recorded for this precursor lies at 3000 cN.

Table 18. mechanical properties and recorded density of samples of precursor a.

Rapid conversion of Pre-stabilized precursor A to carbon fiber

The pre-stabilized fiber candidate "a-3200" was selected for the test involving continuous stabilization in air and subsequent carbonization in an inert atmosphere. The precursor had a degree of reaction of 26%. Using the setup as shown in fig. 11, 4 heating zones of different temperatures were used to stabilize the fibers. Only one pass per zone is required. The residence time in each zone was 5 minutes. The total residence time for oxidation was 20 minutes (4 single passes of 5 minutes of heating). The color did change very rapidly from orange to dark brown in a single pass of 5 minutes when the pre-stabilized precursor was heated in air, demonstrating a highly catalyzed system. After stabilization, the fibers were carbonized using a low temperature furnace and a high temperature furnace saturated with nitrogen. The process parameters used during the experiment are shown in table 19.

TABLE 19 Process parameters for stabilizing and carbonizing fiber "A-3200".

Carbon fibers were produced from this test and the mechanical properties are shown in table 20.

TABLE 20 mechanical properties of carbon fibers produced from precursor "A-3200".

Precursor B

Pre-stabilization in a nitrogen atmosphere

Precursor B fibers were stabilized under nitrogen at different tensions (table 17).

FT-IR analysis was performed and again the maximum value of the degree of cyclization reaction is highlighted. In this case, maximum nitrile cyclization (24%) occurred at 1600cN of applied tension (fig. 16). The mass density is also highest for such tensions.

Mechanical testing was performed on different groups of pre-stabilized fibers. The results are shown in table 21 and fig. 18. The ultimate tensile strength and tensile modulus for the different fiber samples showed a minimum at 1600 cN. The mechanical properties reported are lowest when the content of cyclized nitrile groups is at a maximum. This behavior has been observed for the different precursors tested in this study.

Table 21. mechanical properties and recorded density for samples of precursor B.

Rapid conversion of Pre-stabilized precursor B to carbon fiber

Candidate "B-1600" was manufactured in larger quantities to allow for industrial testing. The fiber had a measured degree of reaction of 24%. The pre-stabilized fibers were successfully stabilized in air. Again, the total residence time in an oxygen-containing atmosphere is equal to 20 minutes. After stabilization in oxygen, the fibers were carbonized in an inert atmosphere using two furnaces. Details regarding the process parameters used during the experiment are shown in table 22.

TABLE 22 Process parameters for stabilizing and carbonizing fiber "B-1600".

Carbon fibers were produced from this test and the mechanical properties are shown in table 23.

TABLE 23 mechanical properties of carbon fibers produced from precursor "B-1600".

The above experiments demonstrate that a variety of different stabilized PAN precursors can be produced rapidly, and that fully stabilized precursors can be successfully converted to carbon fibers, providing mechanical properties tailored for high volume automotive applications.

Example 9Study comparing precursor stabilization process versus rapid precursor stabilization process in the production of carbon fibers with similar tensile properties

In this example, a comparative process representative of a conventional precursor stabilization process used in the industry is compared against the rapid precursor stabilization process of the present invention.

Two industrial trials were carried out using the same precursor feedstock (commercial PAN 24K containing acidic comonomer). Carbon fibers with similar mechanical properties are produced. Furthermore, to allow further comparison of Oxidized PAN Fibers (OPFs) produced by the two processes, a similar density of the stabilized precursor (OPF) was targeted. The fibers produced with the comparative process were stabilized over 96 minutes. Conversely, fibers made with the rapid stabilization process are stabilized in an oxygen-containing atmosphere within 20 minutes. To allow inspection of both processes, fiber samples were taken at the end of the oxidative stabilization and carbonization.

9.1 processing conditions for comparative conventional Processes

Commercial PAN fibers are continuously converted to Carbon fibers using the Carbon fiber production line of australian Carbon Nexus. The PAN fibers were stabilized using four center-to-end ovens that provided four temperature zones with multiple channels in each temperature zone. For oxidative stabilization, the fibers are heated stepwise under tight control of process parameters (tension, line speed, air flow, gas extraction, etc.). For this comparative oxidation process, the precursor fiber residence time was 24 minutes per oven, resulting in a cumulative residence time of 96 minutes. The setup for performing the comparative oxidation process is shown in fig. 5. The tension is adjusted by varying the speed of rotation of the drive rollers on the carbon fiber production line. The process parameters for stabilization are summarized in table 24.

Table 24 process parameters used to compare conventional precursor stabilization processes.

The stabilized fiber is continuously carbonized in an inert atmosphere in two furnaces at low and high temperatures. The furnace was purged with high purity nitrogen to avoid any oxygen contact with the fibers. Respectively, the low-temperature carbonization was performed at a constant tension of 1200cN from 450 ℃ to 850 ℃, and the high-temperature carbonization was performed at a constant tension of 2200cN from 1200 ℃ to 1500 ℃. The total residence time for carbonization was 3.1 minutes.

9.2 processing conditions used in the case of the Rapid stabilization Process

For this continuous process, nitrogen pretreatment was performed using an industrial scale furnace. The furnace comprised 4 controllable heating zones, input and output nitrogen seals and a nitrogen cooling chamber (fig. 1). The furnace was purged with high purity nitrogen and the fiber was heated from zone 1 to zone 4 at temperatures of 285 deg.c and 295 deg.c for the first two zones and the last two zones of the furnace, respectively. For nitrogen pretreatment, the fiber residence time was 1 minute 10 seconds under heating. The test was performed at 1600cN under tight control of tension. The measured degree of cyclization reaction (% EOR) for the nitrogen-pretreated fibers was 23%.

The nitrogen pretreated fibers were further stabilized in an air atmosphere using the same equipment as used for the comparative conventional process (fig. 5). The line speed was initially set to allow a residence time equal to 5 minutes for each zone, thus 20 minutes total (fig. 11). The process parameters used during oxidation are summarized in table 25.

TABLE 25 Process parameters for the rapid precursor stabilization process.

For comparison purposes, the stabilized fibers were carbonized using the same experimental setup as used for the comparative conventional process. Again, the total residence time for carbonization was 3.1 minutes.

9.3. Results

Fibers from both processes were characterized using x-ray, FT-IR, DSC, density column, and tensile testing techniques. The results from each characterization technique of PAN precursor, Oxidized PAN Fiber (OPF), and carbon fiber are summarized in tables 26, 27, and 28.

PAN precursor fiber

Table 26 material properties of PAN precursors.

Oxidized PAN fiber

Table 27 material properties of oxidized PAN fibers extracted from a comparative conventional process and a rapid stabilization process.

Carbon fiber

Table 28 material properties of carbon fibers extracted from a comparative conventional process and a rapid stabilization process.

9.4 discussion

X-ray analysis

The structural composition of the initial PAN precursor, stabilized precursor and carbon fiber was characterized using X-ray spectroscopy. The stabilized fibers and carbon fibers from the comparative conventional process and the rapid stabilization process were analyzed for their crystal structure. In this study, the apparent crystallite sizes Lc (002) and Lc (100) were determined using the Scherrer equation (equation 4). When analyzing carbon fibers, it is observed that the apparent crystallite size Lc (002) is at least 20% larger in the case of a fast stabilization process, although the fibers take significantly less time in heating (21% residence time compared to a comparative conventional process). This finding highlights the different crystal structures adopted by the fibres induced by the different chemical compositions of the stabilised fibres by passing through a rapid stabilisation process.

In addition to the differences observed in the case of carbon fibers, significant differences were also observed in the case of stabilized fibers extracted from both processes. The stabilized precursor fiber extracted from the rapid stabilization process has an apparent crystallite size Lc (002) that is 20% less than the apparent crystallite size Lc (002) observed in stabilized precursor fibers formed by a comparative conventional process. In contrast, no distinguishable difference is seen in the apparent crystallite size Lc (100). The d-spacing between the crystallographic planes of the stabilized and carbonized samples was also analyzed using equation (5). The d-spacing (100) is similar for the stabilized fibers extracted from both processes. It is noted that the d-spacing (002) of the stabilized precursor fibers formed by the rapid stabilization process is slightly larger (4%) than the d-spacing (002) observed for the stabilized precursors formed by the comparative process.

FT-IR spectroscopy

The chemical composition of the stabilized fibers was characterized using FT-IR techniques. The degree of cyclization reaction was calculated for the stabilized fibers extracted from both the comparative conventional process and the rapid stabilization process (equation 2). The stabilized precursor fibers produced by both processes were characterized by similar% EOR, which highlights that the same content of ring structures was formed despite the significantly different time frame for stabilization.

In addition to% EOR, a significant difference was observed when calculating the dehydrogenation ratio (equation 3). The stabilized fibers made with the rapid stabilization process have a dehydrogenation ratio that is at least 13% higher than compared to the conventional process, demonstrating a higher degree of oxidative chemical reaction or higher chemical conversion of the polymer backbone.

Differential Scanning Calorimetry (DSC)

The thermal behavior of the stabilized fibers from the comparative conventional process and the rapid stabilization process was studied by DSC analysis. The CI index of the stabilized fiber was calculated (equation 1) and found to be significantly higher (+ 16%) in the case of fibers processed by the rapid stabilization process. Although the fibers have spent less time in heating, the exothermic conversion is excellent, which highlights enhanced fiber processing.

Thermogravimetric analysis (TGA)

The evolution of weight loss with temperature of the stabilized precursor fiber was analyzed by TGA. The samples were tested under nitrogen at a heating rate of 10 deg.C/min. No significantly distinguishable difference was observed between the stabilized precursors formed by comparing the conventional process and the rapid stabilization process. The stabilized precursor fibers extracted from both manufacturing processes showed similar weight loss at 600 ℃ (25.5 ± 0.7 versus 24.1 ± 0.9 for the comparative conventional process and the rapid stabilization process, respectively).

Mass density

The mass densities of PAN precursor and oxidatively stabilized precursor fiber (OPF) obtained from the two stabilization processes were determined using density column techniques. As explained in the above section, the density of the stabilized fibers is similar (standard for comparison).

However, in carbon fibers formed from differently processed stabilized precursors, although there was significantly less processing time in heating during oxidation for the rapidly stabilized precursors, the density (similar tensile properties) of carbon fibers produced from stabilized precursors produced by the rapid stabilization process was higher than the density (1.774 ± 0.003 versus 1.798 ± 0.005, respectively) of carbon fibers produced from stabilized precursors produced by a more conventional process, demonstrating a different structure/chemical conformation.

Tensile testing by FAVIMAT

Tensile properties of PAN fibers, stabilized precursor fibers (OPF) and carbon fibers were measured using the FAVIMAT technique. As a standard for this example, carbon fibers with similar mechanical properties were produced by two stabilization processes. The tensile properties of OPFs from both processes are similar.

Example 10The effect of the concentration of cyclic structures in the pre-stabilized precursor on the oxidative stabilization.

In this study, the effect of the degree of cyclization of the nitrile group in the pre-stabilized precursor on the subsequent oxidative chemistry was investigated. Three sets of pre-stabilized fibers (PAN precursor 12K) with different levels of% EOR of 17%, 24% EOR and 28% EOR were selected based on their content of cyclic structures as determined by measuring the degree formula of the cyclization reaction using FT-IR technique.

For these experiments, three sets of nitrogen pretreated fibers were stabilized in oxygen using exactly the same experimental conditions. Pretreated fibers Carbon fibers from Australian Carbon Nexus were used in an oxygen-containing atmosphereThe center-to-end oven of the production line was continuously stabilized with the setup as shown in fig. 11. Samples of the precursor fibers were extracted after oxidation in an oven for multiple residence times. Specifically, the precursor fibers were extracted in an oxygen-containing atmosphere after heating residence times of 3.75 minutes, 7 minutes, 10.75 minutes, and 15 minutes. The selected constant tension and temperature for the stabilization experiments were 2200cN and 230 ℃ respectively. The oxidized PAN fibers were tested using FT-IR technology. Thus, the extent of cyclization and CH/CH₂The ratios are calculated by equation 2 and equation 3, respectively.

10.1 discussion

CH/CH₂The evolution of the ratio as a function of heating oxide residence time is shown in fig. 19.

As seen in fig. 19, nitrogen pretreatment of different pre-stabilized precursor fibers with different degrees of cyclization can result in high chemical reaction conversions. CH/CH₂The ratio illustrates the chemical composition of the polymer backbone. With the progress of oxidative chemical reactions (dehydrogenation, oxidation, etc.), it is expected and accepted that the CH species and CH₂The ratio of species will be modified during the oxidation stabilization process, resulting in growth of CH species and CH₂Reduction of functional groups. After nitrogen pretreatment, the fibers were observed to become activated and very reactive to an oxygen-containing atmosphere. The increase in the ratio was greatest for all groups of pretreated fibers just after short exposure of the fibers to heat and oxygen (0-4min area). After this sharp increase, the system becomes more saturated, resulting in a decrease in the rate of oxidative chemical reactions. Note that the most drastic evolution was observed in the case of the pre-stabilized precursor fiber, which initially had the highest percentage of ring structures. The extent of the oxidative chemical reaction is directly dependent on the content of cyclic structures generated in the pre-stabilized precursor. In addition to the substantial impact on the oxidation chemistry, pretreatment of the precursor in nitrogen also enhanced the growth of the nitrile group cyclization reaction, as shown in fig. 20.

Similar to CH/CH₂Evolution of the ratio, just after exposure to heat and an oxygen-containing atmosphere, was observedThe most vigorous ring structure grows. The difference in reaction rate after 15 minutes of oxidative stabilization between the different pretreated fibers was significant. A high% EOR rate (66.7%) was observed with pre-stabilized precursor fibers with 28% EOR after a short exposure of 15 minutes. In the previous examples, the% EOR of the precursor sample stabilized at the end of the oxidative stabilization (OPF sample) was about 70%. This finding was surprising in view of the short time (15 minutes) and low temperature (230 ℃) at which these experiments were carried out for the oxidative stabilization of the precursor. It was concluded that a high cyclization conversion of PAN precursor during nitrogen pretreatment is a prerequisite for rapid oxidation.

Example 11Production of carbon fibers from low-density oxidatively stabilized precursor fibers

The precursor used in this example was a commercial PAN tow comprising 12,000 filaments (12K) comprising an acidic comonomer. In this example, two sets of conditions were recorded, illustrating the production of carbon fibers from oxidized PAN fibers having a very low density, however, this is still acceptable and allows the production of high performance carbon fibers.

For these experiments, the fibers were pretreated in nitrogen into the apparatus described in the previous example (fig. 1) at a temperature of 290 ℃, a tension of 1200cN (tension for optimum% EOR) and a residence time in nitrogen atmosphere of 1 minute 30 seconds. The degree of cyclization reaction (% EOR) of the pre-stabilized fiber (PSN-6) was determined by FT-IR and was 23%.

Two industrial trials (example 11A and example 11B) for carbon fiber production using pre-stabilized precursors were performed using different experimental conditions. Table 29 and table 30 summarize the process parameters for example 11A and example 11B, respectively. For both tests, three center-to-end ovens from the production line of Carbon Nexus, australia were used to rapidly stabilize the pre-treated fibers, the ovens being set to provide three temperature zones for oxidation of the pre-stabilized precursor fibers. The total residence time in the oxygen atmosphere (oxidation) was 15 minutes. After oxidation, the fibers were carbonized under tension in an inert atmosphere using similar experimental conditions, as shown in tables 29 and 30. The cumulative carbonation residence time was 3.9 minutes. The fibers were extracted after each zone of the process and tested using FAVIMAT.

TABLE 29 Process parameters used in example 11A.

TABLE 30 Process parameters used in example 11B.

Tables 31 and 32 categorize the evolution of mechanical properties, mass density and degree of cyclization reaction along the entire manufacturing process.

Table 31. evolution of mechanical properties of PAN fibers through the entire production process of example 11A.

Table 32. evolution of mechanical properties of PAN fibers through the entire production process of example 11B.

High performance carbon fibers were observed to be produced by two industrial trials. However, for both examples, it is noted that the density of the oxidized PAN fiber was 1.313g/cm for example 11A and example 11B, respectively³And 1.336g/cm³. These density values are considered low because the density range of the stabilized precursor fibers known for producing high performance carbon fibers (OPF fibers) generally mentioned in the literature is generally in the 1.340g/cm range³And 1.390g/cm³In the meantime. For both examples, the content of the ring structure of the stabilized fiber (region)3) Similar to that measured with baseline example CE-1. For example 11A and example 11B,% EOR was 66.6. + -. 0.7% and 67.8. + -. 0.4%, respectively. Although the stabilized fibers are less dense, the reinforced chemical structure formed using this process allows for efficient formation of high performance carbon fibers.

Example 12Examples of determining the degradation temperature of PAN precursors

The degradation temperature of PAN precursors was tested using Differential Scanning Calorimetry (DSC). The degradation temperature is specified as the maximum of the exothermic transition of the PAN precursor that occurs under a nitrogen atmosphere. Three milligrams of precursor fiber were tested by DSC under a nitrogen atmosphere at a heating rate of 10 ℃ per minute.

Table 33 illustrates the degradation temperatures of some PAN precursors used in some embodiments herein.

Table 33. process parameters for stabilization (compare conventional process).

Precursor numbering	Usage in the examples	Degradation temperature (. degree.C.)
			1	8 (precursor B),9	303.2±0.7
2	8 (precursor A)	300.6±0.9
			3	13	311.9±1.5

Figure 21 shows DSC traces of different precursors to illustrate the degradation temperatures of the precursors.

Comparative example 2(CE-2)Attempts to rapidly stabilize PAN precursor fibers with a pre-stabilized precursor having less than 10% cyclized nitrile groups

In this example, a commercial PAN precursor comprising an acidic comonomer was used. The precursor fibers were pre-stabilized in nitrogen using the equipment previously used in example 2. For all heating zones, the temperature was set to 280 ℃. The line speed was set to provide a heating residence time of 6 minutes for pre-stabilization. After pre-stabilization, the coloration of the fiber changed from white to orange. Although the change in colour highlights the chemical change, the pretreated fibres were subjected to FT-IR analysis and gave low degree cyclisation values of 6.1. + -. 0.8% EOR.

A rapid stabilization experiment was attempted on the pre-stabilized precursor fiber using a single oxidation oven (fig. 9). For this example, the heating residence time in the oxidation oven was 15 minutes. The rapid stabilization experiments were first performed at temperatures ranging from 230 ℃ to 260 ℃. In this example, although the partially stabilized precursor has been exposed to oxygen, it was found that the resulting precursor was not sufficiently thermally stable after oxygen exposure, as determined by flame testing. That is, when the open flame is held on the precursor after exposure to oxygen, the precursor cannot adequately withstand the open flame without burning or significant smoldering. Thus, oxygen-treated precursors are considered to have unsatisfactory carbonization quality.

After these preliminary experiments, the temperature inside the oxidation oven was set to a higher temperature of 270 ℃. However, this higher oxidation temperature leads to degradation of the pre-stabilized precursor fiber material due to excessive heating rates, uncontrolled management of exothermic reactions, and insufficient chemical production in the pre-stabilized fibers (i.e., not enough nitrile group cyclization).

It is to be understood that various other modifications and/or alterations may be made without departing from the spirit of the present invention as outlined herein.

Claims

1. A process for preparing a stabilized precursor, the process comprising:

exposing the pre-stabilized precursor to an oxygen-containing atmosphere to form the stabilized precursor.

2. The process of claim 1 wherein the precursor is heated in the substantially oxygen-free atmosphere for a period of time of no more than about 5 minutes.

3. The process of claim 1 or claim 2, wherein the precursor is heated in the substantially oxygen-free atmosphere for a period of time of no more than about 3 minutes.

4. The process of any one of the preceding claims, wherein the precursor is heated in the substantially oxygen-free atmosphere at a temperature no more than 30 ℃ below the degradation temperature of the precursor.

5. The process of any one of the preceding claims, wherein the precursor is heated in the substantially oxygen-free atmosphere at a temperature in the range from about 250 ℃ to 400 ℃.

6. The process of any one of the preceding claims, wherein the precursor is heated in the substantially oxygen-free atmosphere at a temperature in the range from about 280 ℃ to 320 ℃.

7. The process according to any one of the preceding claims, wherein the amount of tension applied to the precursor is selected to promote the formation of at least 15% of cyclized nitrile groups in the pre-stabilized precursor as determined by fourier transform infrared (FT-IR) spectroscopy.

8. The process according to any one of the preceding claims, wherein the amount of tension applied to the precursor is selected to promote the formation of about 20-30% of cyclized nitrile groups in the pre-stabilized precursor as determined by fourier transform infrared (FT-IR) spectroscopy.

9. The process according to any one of the preceding claims, wherein the precursor has the potential to reach the maximum amount of cyclized nitrile groups, and the amount of strain applied to the precursor is selected to promote up to 80% less cyclization than the maximum attainable nitrile groups in the pre-stabilized precursor.

10. The process according to any one of the preceding claims, wherein the precursor has the potential to reach the maximum amount of cyclized nitrile groups, and the amount of tension applied to the precursor is selected to promote the maximum nitrile group cyclization in the pre-stabilized precursor.

11. The process of any one of the preceding claims, wherein the amount of tension applied to the precursor is in the range from about 50cN to about 50,000 cN.

12. The process of any one of the preceding claims, wherein the substantially oxygen-free atmosphere comprises nitrogen.

13. The process of any one of the preceding claims, wherein the pre-stabilized precursor is exposed to the oxygen-containing atmosphere for a period of time of no more than about 30 minutes.

14. The process of any one of the preceding claims, wherein the pre-stabilized precursor is heated in the oxygen-containing atmosphere at a temperature lower than the temperature used to form the pre-stabilized precursor.

15. The process of any one of the preceding claims, wherein the pre-stabilized precursor is heated in the oxygen-containing atmosphere at a temperature in the range from about 200 ℃ to 300 ℃.

16. The process of any one of the preceding claims, further comprising the step of cooling the pre-stabilized precursor before it is exposed to the oxygen-containing atmosphere.

17. The process of any one of the preceding claims, wherein the stabilized precursor is formed within a time period of no more than about 30 minutes.

18. The process according to any one of the preceding claims, which is a continuous process.

19. The process of any one of the preceding claims, wherein the precursor is in the form of a fiber.

20. The process of any of the preceding claims wherein the stabilized precursor is formed at an average energy consumption in the range of from about 1.1kWh/kg to 2.6 kWh/kg.

21. The process of any one of the preceding claims, further comprising the step of determining a tension parameter for a precursor prior to forming the pre-stabilized precursor, wherein determining the tension parameter for the precursor comprises:

(b) applying a series of different substantially constant amounts of tension to the precursor while heating the precursor in the substantially oxygen-free atmosphere at a selected temperature and for a selected period of time;

(c) determining the amount of cyclized nitrile groups formed in the precursor for each substantially constant amount of strain applied to the precursor by fourier transform infrared (FT-IR) spectroscopy;

(d) calculating the tendency of the degree of cyclization of the nitrile group (% EOR) with respect to the strain;

(f) the amount of strain that causes at least 10% of the nitrile groups to cyclize is selected so as to pre-stabilize the precursor.

22. The process of claim 21, comprising selecting an amount of strain that causes maximum nitrile group cyclization to pre-stabilize the precursor.

23. A process for making carbon fibers comprising the steps of:

providing a stabilized precursor fiber prepared according to the process of any of the preceding claims; and

carbonizing the stabilized precursor fiber to form the carbon fiber.

24. The process of claim 23, wherein carbonizing the stabilized precursor comprises heating the stabilized precursor in an inert atmosphere at a temperature in a range from about 350 ℃ to 3000 ℃.

25. The process of claim 23 or claim 24, wherein the carbon fibers are formed in a time period of no more than about 70 minutes.

26. The process of any one of claims 23 to 25, wherein the carbon fibers are formed in a time period of no more than about 45 minutes.

27. The process according to any one of claims 23 to 26, which is continuous and comprises the steps of:

feeding a precursor comprising polyacrylonitrile to a pre-stabilization reactor comprising a substantially oxygen-free atmosphere, and heating the precursor in the substantially oxygen-free atmosphere while applying a substantially constant amount of tension to the precursor to promote cyclization of nitrile groups in the precursor, the temperature and time period at which the precursor is heated in the substantially oxygen-free atmosphere and the amount of tension applied to the precursor each being selected to form a pre-stabilized precursor comprising at least 10% cyclized nitrile groups as determined by fourier transform infrared (FT-IR) spectroscopy;

feeding the stabilized precursor into a carbonization unit, and carbonizing the stabilized precursor in the carbonization unit to form the carbon fiber.

28. The process of claim 27, further comprising the step of cooling the pre-stabilized precursor prior to feeding the pre-stabilized precursor to the oxidation reactor.

29. A low density, stabilized precursor fiber comprising polyacrylonitrile having at least 60% cyclized nitrile groups and having a nitrile content of from about 1.30g/cm³To 1.33g/cm³Mass density in the range of (1).

30. The low density, stabilized precursor according to claim 29, having at least 70% cyclized nitrile groups.